I wouldn't without a lot of data in front of me (or likely at all). I don't shoot muzzle loaders, but a quick search online shows a 300 gr .50 cartridge at somewhere between 4,000-15,000 PSI depending on the type and quantity of powder being used. .500 S&W Shows a max somewhere between 51,000 and 62,000 (some conflicting information). The rim diameter is also .050" larger than .500, so you're going to have more bearing surface for the pressure. That's an enormous increase.
You may want to find the thread on here of the member who blew up his T?C in .300 Win Mag with loads over max pressure. He was just using over pressure loads in the cartridge the gun was designed for and caused damage to himself and his rifle when it couldn't contain the pressure.