I'm about to build a .358 smokeless just to see what it will do. Should have amazing ballistic properties for a muzzleloaders.
I figured someone had already tried or thought of trying just what you are wanting to do.
My next thought is what kind of bullet are you planning on using. Are there any sub sized sabots that would accommodate a real rifle bullet? I'm thinking it would be nigh on impossible to muzzle load a copper jacketed bullet.