Interestingly enough, the idea of muzzle loading the shot with appropriate wads and whatnot occurred to me almost immediately after a minor mishap in testing. I had just filled and capped the first capsule I made when I let it roll off my workbench in a moment of inattention and it snapped right in the middle. The single-layer print is plenty fragile enough to disintegrate upon firing, but it's also so long and thin that it just isn't up to much abuse. Since changing the diameter isn't an option, it seemed obvious to me that getting rid of the capsule entirely might be an option.
I haven't done any additional experimenting with that yet, part of me wants to figure out how to deal with the donut pattern first. If that happens, I am very interested in cartridge-powered smokeless muzzle loading shot...shell? Shot loads? Errr... Yeah....