In order to prevent becoming Bubba's 'stress ball' for the next 10 years and forking over up to 250,000 in fines, it's best to do a Form 1, pay the $200 tax get ATF approval and then, only after you get approval, begin construction. The ATF has long ruled that airgun silencers, even those that should theoretically not function with a firearm are in fact suppressors and require an NFA tax stamp. In fact, tyo my knowledge, they have never once approved an airgun silencer design as not requiring a tax stamp.
On a practical level, I think this would be a costly, and relatively not well thought out endeavor. A muzzle loader would leave a lot of corrosive substances in the suppressor. Unless it was very easily detail stripable (read, entirely disassembled), the thing would probably be ruined in no time, since you would be leaving corrosive residue in the baffles, which would be not helpful.
Maybe the project would be better suited for one of those smokless savage ML"s or something...