I think his point was that it just wouldn't be right to use modern metallic cartridges as an integral part of something made to appear historically accurate to a time well before modern metallic cartridges were popular.Don't think you need to seal the horn so much as the wood. Don't worry about the brass around ML stuff, its fine. All the powder measures are made of brass. I've actually seen a video of a static electric spark going around and through a pile of black powder and it not detonating. Can't light BP with static.
An antler tip (turned down some)might make a good stopper.