yeah they do make clear Powder coat... Not sure how well it would work on your expansion chamber. It can generally withstand temps in the 400's.
Now the question is this... Are you going to be putting this on a raw steel pipe, a chromed pipe, painted pipe, or a previously pc'd pipe?
Raw steel will get you the best result, Chromed pipe will not work out very well since the clear is only as good as the chrome underneath, painted would not work at all.... A previously powder coated pipe (fresh coated) will probably be ok, but I would think using High temp powder would be the way to go with that.