You might check out these guys. I've read lots of good comments on their work on the FFR forum and I'm getting ready to ship my header pipes, collector/headpipes and mufflers off to them. They coat both inside and outside. I would send my tailpipes to them (undercar exhaust) but I don't want to mess with trying to ship them oversize. They also do a satin black ceramic coating which is what I'm having done.
High Performance Ceramic Header Coatings - Nitroplate