Realize harmonica is not "metal" but this site does still discuss music on the occasion. Only ones I know of really are John Popper (Blues Traveler), Babe Pino, and Magic Dick (J Geils Band.) I'm sure theres many others worth checking out.
have a couple harps sittin around the house. my gf is trying to learn so we can jam on some blues together. would like to figure it out too. need inspiration and such.
James Cotton might be up your alley. I had a compilation CD with him on it at some point. Mixes vocals in with his harp playing. Outdoor whiskey drinking music.