There isn't an air space issue in muzzle loading cannon. The projectiles are not bore tight as in muzzle loading/blackpowder cartridge rifles and are seated up against the charge when loading, no matter the size of charge.
When fired cannon shells are not bumped up to seal the bores like in rifles. The powder only has to overcome inertia and not friction. In fact cannon projectiles are supposed to have windage, and be able to slide the length of the bore by gravity alone. If you review the safety rules posted at the top of this forum from the Artillery Association and N-SSA you will see they are adamant on this point.
In the Mortar, the chamber is the barrel also. The portion holding the projectile is nothing more than an expansion chamber. A reduced charge might leave an air space between charge an ball. I have neve been concerned with this as the front of the cartridge is built up from the folded excess tin foil and fills the chamber.
With windage the expanding gases can blow by the projectile, both in the mortar and cannon.
Corn meal should work if you are concerned about airspace.
While we are on the subject. I asked last week when I was at Winchester picking up powder, why cannon balls were not patched for accuracy.
The answer was Safety.
If you look at the safety rules you will see a step called "thumb the Vent. This is done during the loading process.
The vent is covered with the thumb so no air can rush out. If there was a smouldering ember still in the barrel when you rammed home the charge it could flare up from the draft of escaping air igniting the charge prematurely. This could be disastrous if the charge or projectile was part way down the barrel. The loader or other persons working the gun could be injured.
Of course it would virtually impossible to seat a patched projectile with a sealed vent. The windage being just a space around the projectile for the air being compressed to escape.
