Try a 10 minute soak with MAP. (Murphys, Alcohol and Peroxide) The peroxide component will dissolve the fouling and then it will rinse out.I haven't tried it as a patch lube, or for anything regarding a muzzleloader, as I don't know if it contains petroleum products.
I have used it, sparingly, on unmentionable guns. It works great for cleaning the worst of bored, but, I say sparingly because it smells like it is killing you with every inhalation while using it. But it cleans incredibly well.
I also don't really see a reason to use it as a "lube" as it isn't that slick and there are better, cheaper, more proven things, readily available.
It is funny you bring it up now though, as I have been thinking lately of trying it as a bore solvent in my smoothie. Since my last shooting session i have a tiny bit of black crud visible around the corners of where the breech plug meets the barrel walls. No amount of water, hot water, hot water with dawn, and use of breech scraper seems to be removing it. Never have I had this issue before, but, this is one of the very few times I've ever swabbed between shots. Was thinking of taking advantage of Butch's excellent cleaning ability, then many, many more hot water rinses.