Somehow, I'm reminded too much of Gilbert and Sullivan – "let the punishment fit the crime tra la, but the punishment fit the crime."
If you are going to use solid ball out of the muzzleloader, why not just get a rifle board for the ball? Years ago, my uncle made a double barreled caplock rifle in 75 caliber. The thing has 54 inch barrels that are sighted for impact at 75 yards. My husband has been using it to hunt deer ever since he shamelessly stole it from me after his discharge. He could easily use the flintlock 12gauge but would it have the accuracy not to mention the energy potential of that gawd-awful heavy rifle?