I had my exhaust manifolds ceramic-coated and I used the black POR-15 on my rusted battery tray. The POR-15 instructions say it dries harder than ceramic, which I will say appears to be very true, so if the gray POR-15 says it will take the high temperatures of the exhaust system, I'd say give it a shot.
I bought a small "starter kit" at NAPA that had all three components (cleaner, metal-ready and 8oz. of final coating) for under $20, and I still have most of it left after 2 coats on my battery tray and a layer of protection in the battery area of my engine compartment and frame under the battery tray. It's expensive by the quart or gallon compared to regular paint but a little goes a long way.
