I just got a price from Finishline Coatings to coat my Hedmen headers - they are about $250 for the pair.
They coat the inside and the outside - and will withstand temperatures up to 2000 degrees.
I am taking mine in this week to drop them off to be coated. I will let you know how they turn out. From what I have gathered from talking with them and from receiving another recommendation from a local shop, they seem to be the place to go around here for this kind of thing.