There has to be some black powder cartridge people around that give you the the real scoop.
I don't think you can compress black powder too much, however, I could change my mind if some experts tell me different.
I've been loading 45-70 cartridges with pryodex rs. The 10th edition of the Hornady reloading handbook has a recipe for an original Springfield Trapdoor and even the lowest amount nearly fills the case.
Would a gas check on your bullet instead wad give you a little more room to seat the bullet? Just a "hip-shot" idea.