Nothing.
The projectile sat on top of the gunpowder and was glued to the paper, as demonstrated by Guns of the West.
Because the paper cylinders were typically rolled using a fixed size dowel, if you were using a reduced charge you could use a filler such as flour or any inert powder, but that was only to ensure there was no gap between the ball and the powder. Such a gap might make the already fragile paper cartridge even more fragile.
The question supposes two problems which might be solved by adding a wad to paper cartridges.
- Reducing heat on the projectile to reduce lead from being deposited inside the barrel (reduce "lead fouling").
- Seat the projectile firmly against the barrel to improve accuracy (reduce "cylinder gap").
I don't believe paper cartridges attempted to solve either issue.
Cylinder gap
The issue of keeping the projectile snuggle against the rim of the cylinder was not solved by revolvers at the time. Revolvers will always have a cylinder gap. Even modern metallic cartridge revolvers with higher tolerances still have the issue; a notable exception being the 1895 Nagant Revolver. This is also why revolver rifles were a bad idea, your support hand could easily be harmed by gas leaking out of the cylinder, as demonstrated by Ian McCollum on a Colt 1855 Revolving Rifle.
Back to the question. Here is Karl Kasarda demonstrating loading a 1862 Colt Police Revolver with paper cartridges. Like in any muzzle loader, the chambers of the cylinder are not bored-through; they have a solid back. Karl uses the loading lever just like a ramrod; he rams the cartridge into the chamber from the muzzle end to firmly seat the projectile in the chamber, and compress the gunpowder against the bottom of the chamber.
This is both to prevent the projectile and gunpowder from falling out, and also keeps the gunpowder snug against the primer to ensure the gun fires. The quantity of gunpowder used does not matter, it will be compressed against the back of the chamber by the loading lever.
This loading procedure pushes the projectile away from the cylinder edge defeating the premise of the question.
Lead fouling
This was a real thing, and again, wasn't solved by revolvers of the time.
There are typically two causes of lead fouling. One is, yes, a tiny amount of lead being turned into gas by the explosion of the gunpowder. The other is a tiny amount of lead being scraped off the projectile as it transitions from the chamber to the barrel.
The issue of depositing lead in the chamber or barrel (fouling) could be handled by applying lubricants to the paper cartridge, which had their own fouling issues, but the reality is black powder fouling was a far worse issue. Black powder guns had to be cleaned regularly regardless of lead fouling, as often as after a couple dozen shots.
You could add a wad between the powder and the projectile to attempt to reduce fouling, but it was not necessary, and it was not to reduce heat on the projectile. It was to better seal the chamber and avoid having gas and unburnt powder from leaking out around the projectile.
What is wadding?
There is something similar to what you're asking about: wadding or a wad. Modern projectiles are carefully matched to the barrel diameter and serve to seal the barrel. But because the projectiles of the time were often irregular and slightly sub-caliber. This was both to ease their loading down the barrel, and also so they would still work after the barrel had become fouled and thus narrower.
When the projectile does not fully seal the barrel, wadding is necessary. Without a wad, expanding gas and unburnt powder could leak out around the projectile. This would waste the force of the powder, and also add to fouling. Shotguns loaded with multiple sub-caliber pellets (buckshot, birdshot) still use them today.
All those sparks coming out of movie guns might look cool, but it's unburnt powder. It betrays that there's no projectile in Ash's Boomstick to seal the barrel. His "recoil" is the actor flinching and exaggerating.
The reality is you can use anything as a wad. Karl Kasarda likes using coffee filters, and even leaves and twigs.
What is a paper cartridge?
To make it clear, paper cartridges for revolvers are different from those for smoothbore muskets (long arms) and different again for breechloading rifles.
For a long arm, the paper cartridge served as a pre-measured portion of gunpowder and projectile(s) to speed loading and make it more precise than fumbling around in a pouch for a projectile and pouring gunpowder from a flask. The cartridge was tied off into two sections separating the gunpowder from the ball. The user would bite the paper open, pour a small portion of gunpowder into the pan, and pour the remaining gunpowder into the barrel. Then they'd wrap the paper around the projectile(s) and ram the wrapped projectile(s) down the barrel. So in that case the paper itself served as the wadding.
For revolvers, the paper cartridge served like a metallic cartridge today, except it lacked a primer; the primer came separately often as a percussion cap. Loading a paper cartridge into a revolver was done in three steps.
- Shove the whole cartridge into the chamber.
- Firmly seat the cartridge in the chamber using a loading lever.
- Place a percussion cap onto the nipple.
