Just found this website - someone used the Cermachrome product to coat an aviation exhaust, about 30 bucks for 6oz for a full system
http://www.techlinecoatings.com/BulkExhaust.htm
(http://www.snowhill.com/~mstumpf/images/muffler1.jpg)
Its a spray or sponge on coating that needs to be baked I think, at 500f.
I've heard about this stuff before. I don't know how well it works compared to having it coated professionally, but I'd guess not quite as well. For that cheap, it may be worth a go
I haven't tried ceramic coating yet but from what I've read on the forums it's really important to coat the inside as well. That's to prevent corrosion from eatting the exhaust from the inside out. Sounds like a good deal if it works.