powder coating is powdered plastic that is heated to congeal and create a coating. Exhaust heat will most likely heat this up past its designed melting point and casue it to burn off or melt off. Ask Leanne...they got special hi temp stuff now, 600-1200F ratings! THAT would work on a manifold. High Temperature Powder Coating Services
You could probably get it ceramic coated or some other thermal barrier that would last. Problem is that if they are desirable manifolds 340 hi-po, bb a body, someone may not want them thermal coated. Otherwise no issues.
Normal cosmetic powder coating is good to 500-600 degrees give or take if the job is done right, but exhaust applications get MUCH hotter than that, especially right at the heads. The best bet is a high temp ceramic coating.