It's not so much that you can't use Hoppes solvent, but that you don't need it...andwill need to buy lots of patches. Black powder products are water soluable, and left behind in significant quantity. That is why the bucket of water, some kind of detergent, and good swabbing. Look at the water after you are done. A few shots from a ML will produce much more crap than you will get from cleaning patches through,say,a standard shotgun after 100 rds on the trap range. And while the newer stuff isn't quite as corrosive as true black powder, leave it too long and it will eat your barrel.
Everyone has their favorite detergent, but I have taken to Simple Green.