I have shot prb from my muzzleloading shotguns with adequate accuracy to consistantly hit a paper plate at 25 yds. The problem I see is that when the ball is pushed down in the brass shell to sit on top of the powder, it is smaller than the bore size. As a result, when fired, it will rattle around while coming out. If you choose to go that route, make sure you have a wad under the ball to keep all the gases from blowing by the ball. Another thing that could be tried is to put a very short brass shell into the gun, then pour powder down and follow it with a patched ball. The powder would have to be enough to fill the short shell so there would not be an air space between the powder and the ball. This would give better accuracy but I personally would not do it since you would be pushing a ball down on a load with the primer in place. It would be like putting on a percussion cap and then proceeding to put in the powder and prb. Just not a safe operation.
However, if you put in the prb and pushed it most of the way down, then holding the barrel vertically, pushed in the short shell full of powder with it fitting snug against the prb, closed the gun, pull back the hammer and fired, it could be done with reasonable safety. You'd have to size the shell to hold the amount of powder needed so there would be no air gaps.
Only issue still might be blowback from the breech. Without an airtight seal at the breech, there could be some blowback so if you decide to try any bp shooting, I'd recommend that the first shots be fired using a string and with the gun held in an old tire, strapped to a tree, etc.