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Simple answer: No

Reverse Gravity affects creatures and objects, and a spell is neither (though a spell may conjure or create a creature or object).

And, as you say, any spell with a duration of instantaneous effectively happens without any chance for something to affect it unless specifically noted.

A more 'physical' argument against Reverse Gravity affecting Meteor Swarm or similar spells is that the spell creates the blazing orbs of fire with enough force and momentum for them to reach their targets before gravity can have much of an effect.

After all, even without Reverse Gravity, a spell that launches something at a target doesn't have to worry about gravity pulling it to the ground before it reaches the target!

OP: Your room is 85 feet tall????

Alright, this is a case where Jeremy Crawford's mantra applies to all Rules as Written answers.

Spells do Exactly What They Say

Reverse Gravity

7th level transmutation

This spell reverses gravity in a 50-foot-radius, 100-foot high cylinder centered on a point within range. All creatures and objects that aren’t somehow anchored to the ground in the area fall upward and reach the top of the area when you cast this spell. A creature can make a Dexterity saving throw to grab onto a fixed object it can reach, thus avoiding the fall.

If some solid object (such as a ceiling) is encountered in this fall, falling objects and creatures strike it just as they would during a normal downward fall. If an object or creature reaches the top of the area without striking anything, it remains there, oscillating slightly, for the duration.

At the end of the duration, affected objects and creatures fall back down.

Gravity is reversed, things fall up. Cool. You're either falling to the ceiling or hanging from the floor. Right? Alright, we all agree here.

Meteor Swarm

9th level evocation

Blazing orbs of fire plummet to the ground at four different points you can see within range. Each creature in a 40-foot-radius sphere centered on each point you choose must make a Dexterity saving throw. The sphere spreads around corners. A creature takes 20d6 fire damage and 20d6 bludgeoning damage on a failed save, or half as much damage on a successful one. A creature in the area of more than one fiery burst is affected only once.

The spell damages objects in the area and ignites flammable objects that aren’t being worn or carried.

Flaming orbs appear and plummet toward the targets. It doesn't say:

• Which gravity it used


• Where the orbs appear


• What inertia they already have from wherever they were pulled.

They appear and plummet toward the targets. If that means they appear "beneath"
them and fall up or above them and fall down or if they magically fall sideways -- because magic -- then they fall sideways. Whatever direction they plummet, they plummet at the targets.

So, if you're hanging from the floor or fell to the ceiling, maybe they appear "above you" closer to the floor than you currently are and traveling in your direction, or they appear already having a huge amount of inertia to overcome the effects of the reverse gravity spell. I don't care what you do narratively to convince yourself and your players why it happens -- the spell says what it does.

From a fairness perspective would you let someone that was spider climbing and clinging to the ceiling allow a 9th level spell to miss them because of the word "plummet"? No, right, because that would be cheap. It's a 9th level spell slot.

Ground

I'm not going to get caught up in definitions of the word "ground" in this context. When gravity is reversed, is "ground" the ceiling or the floor? I don't know and really don't care. It just has to be made to make sense with the narrative. If we keep a tight definition of ground, the spell couldn't ever be cast in open waters, under water, on planes like the Astral Plane or Pandemonium because "ground" doesn't work like we think here on our earth in those places. And that isn't what is intended.

Points, not Creatures

The "a point you choose" don't have to be on the ground. It can be in a person's gut or head or whatever. Moreover, the area of effect for each of the 4 points is the origin of a 40 foot radius sphere which has a diameter of 80 feet. So if the room in question isn't 85 feet tall, the whole argument is moot... because the sphere can encompass both the ceiling and floor. The spell is instantaneous so the person won't have moved by the time the meteor gets there. Unless they have a reaction to use a spell to get out of the way, which, I suspect, is one of the reasons the target is points and not a creatures. Another is so that you can use it on objects. You can't magic missile or eldritch blast a door, but you can meteor swarm it. (Did this really need saying?)

But Physics...

If you're a strict follower of Rules as Written, or even if you aren't, part of the job a DM has is to make the rules make sense in context. Find a flavor or spin that makes the game mechanics work for the understanding of the surrounding environment and physics. So those meteors already have inertia from wherever they were conjured from that is greater than the reverse gravity field, or they conjured closer to the floor so they plummet "upward" toward the characters. "Physics" isn't a good reason to make someone, NPC or PC, lose a 9th level resource for nothing... However...

If reverse gravity is already up, and meteor swarm is cast -- no I'm not going to let a 7th level spell mangle a 9th level casting for no good reason because I think our world's physics should apply to a world where where flaming orbs can just appear, a person waving their arms and saying words can reverse gravity, an expensive enough gem can be used to bring back someone from the dead, and dragons are a real threat to local commerce... D&D is simulations, and our descriptions of what happens as a result of the mechanics is what makes anything in the world believable or not.

Rules as Cool

The rules aren't everything. If a PC readies a held casting of reverse gravity with a trigger of meteor swarm, that's pretty clever. And the order can make a narrative to why the "physics" works and rewarding the player for his creativity and his risk of wasting a 7th level slot if meteor swarm isn't cast is a perfectly acceptable narrative tool. Don't have an NPC do it, but allowing a PC to do that once is not a problem. That said, they'll have to fall 85 feet to get out of the way -- which will still be a huge amount of damage.

The blobs of fire created by Meteor Swarm would be affected by Reverse Gravity

As mentioned Meteor Swarm has a duration of instantaneous. For the purposes of the rules instantaneous has a specific meaning in relation to the spell duration:

Instantaneous

Many spells are instantaneous. The spell harms, heals, creates, or alters a creature or an object in a way that can't be dispelled, because its magic exists only for an instant.

The instantaneous duration refers to the creation of the objects in the spell, the "effect" of the spell. Importantly it does not refer to the resolution of those effects (and in particular the damage caused by the effect).

In the case of Meteor Swarm the effect it creates is

Blazing orbs of fire plummet to the ground at four different points you can see within range.

The key word here is "plummet". This is not a defined game term, thus we must check what it's natural meaning is in English.

Plummet means

to fall very quickly and suddenly:

House prices have plummeted in recent months.

Several large rocks were sent plummeting down the mountain.

She plummeted to the ground.

Falling is something that is explicitly affected by Reverse Gravity

All creatures and objects that aren't somehow anchored to the ground in the area fall upward and reach the top of the area when you cast this spell.

How are objects defined in the rules?

DMG > Running the Game > Objects

For the purpose of these rules, an object is a discrete, inanimate item, like a window, door, sword, book, table, chair, or stone, not a building or vehicle that is composed of many other objects.

• It is discrete (each orb is described as a single item made of a single substance)


• It is inanimate


• It is not composed of many other objects (each orbs isn't composed of other small objects, they are a contiguous whole)

The orbs of fire are pretty clearly objects and thus would be affected by Reverse Gravity when they come into the area of effect of Reverse Gravity.

Even if they are not objects, but are a contiguous whole made up of smaller objects, it doesn't matter. Each of the constituent objects would be affected by the Reverse Gravity field as the orbs of fire are not "anchored to the ground".

Alright, I'm going to answer this directly via physics.
The amount of Physical damage a character is going to get from a meteor depends upon the change in momentum before and after the collision. (I am skipping the damage that will happen due to the fire on the meteor(orb)).
Things to consider:
1) Both, the character and the orb are under the same effects of acceleration due to gravity or anti-gravity.
2) As the mass of both the objects (character and meteor) remain the same in both conditions, (gravity or reverse gravity),$$ text{Mass of character } = m text{ and Mass of meteor } = M $$
CASE 1: Normal gravity:
Velocity of the character is 0: before the spell starts, before the meteor hits and after the meteor hits.
After the spell starts,
$$
Meteor Initial Speed = v_1
$$$$
Acceleration On Meteor = alpha (= g downwards+ some other magical acceleration if added)
$$
After the meteor hits the character,
$$
MeteorFinalSpeed = v_f = CharacterFinalSpeed = 0
$$
Meteor speed just before hitting: $$ v_2 = v_1 + alpha Delta t $$
$$ text{Where } t text{ is the time taken for meteor to reach the character}$$
or, $$ v_2 = root of {v_1^2 + 2alpha Delta h}$$
$$ text{Where } h text{ is the initial distance between the two objects} $$
$$ text{Hence, change in momentum = } - Delta rho = M(v_2 - v_f) = M(v_2 - 0) = Mv_2 $$

Now, those of you who understand relative motion, probably already know the answer, but I'll continue...

CASE 2: Reverse Gravity
$$ text{Initial velocity of character } = u_1`$$
$$ text{Velocity of character just before collision } = u_2`$$
$$ text{Velocity of character just after collision } = u_f`$$
$$ text{Same for Meteor } v_1` v_2` text{and } v_f`$$
$$ text{Acceleration on character }= -g text{ and acceleration on meteor } = alpha ` text{ = g upwards + same magical acceleration as before if added} $$
$$text{Hence, } alpha - alpha ` = 2g $$
$$ u_2` = u_1` - g Delta t text{ and } v_2` = v_1` + alpha ` Delta t $$
$$ implies v_2` - u_2` = v_1` - u_1` + (alpha - 2g)Delta t + g Delta t text{ is the relative velocity just before collision}$$
Keeping that aside, looking at the velocity of the objects just after collision should enable you to get the relative damage as compared to case 1 using what follows:
$$ text{Change in momentum } = - Delta rho `= M (v_2` - v_f`) + m (u_2` - u_f`)$$
To get the damage in case 2 you need to use:
$$ text{Damage in case 2 } = D` = text{Damage in case 1 } (D)text{ x } frac{Delta rho `} {Delta rho} $$
$$ implies D` = D frac{M (v_2` - v_f`) + m (u_2` - u_f`)} {Mv_2} $$
$$ implies D` = D frac{M (v_1` + alpha ` Delta t - v_f`) + m (u_1` - g Delta t - u_f`)} {M(v_1 + alpha Delta t)} $$
Of course that means that the amount of damage depends upon how fast the character is going up. But, by what I see it will most of the times be greater in Case 2 than in Case 1, because the character is actually going upwards, towards the meteor.
Now of course, this is still only the analysis of the vertical component of the velocities, but hey that's what the gravity will be affecting after all.