Howdy
We had a similar discussion over at CAS City just a few weeks ago. I don't shoot muzzle loaders but I shoot Black Powder in cartridge revolvers, lever guns, and shotguns all the time. Old guns from the 1880s right up to modern Stainless guns. Nice shiny modern bores as well as old pitted bores.
I don't know much about 'seasoning', but I can categorically state that once they have gone through the firing and cleaning cycle a few times with my favorite water based cleaning solution and lubricating the bore with Ballistol, they are easier to clean and require less elbow grease to get clean then the first time they were fired with Black Powder. Whether or not you call it 'seasoning', whether or not there are micro pores in the steel, they clean up quicker and easier after having gone through the firing and cleaning cycle a few times.