John
Don't be too quick to discount Rubber.
Some silicones are good to 315 deg C (599 F) and 245,000 ohm-cm.
About a month ago there was a thread on ceramic sources.
Coors Porelain in Golden, Co and at least one other. Ceramic might not be flexible enough for 15" rad. It depends on the cross section of the steel (actually Moment of Inertia) and the Modulus of Elasticity of the ceramic. If it is a very small cross-section, it could work. You might need to thermally spray the coating which I believe is termed plasma spraying. Or you could coat with Aluminum Oxide Al2O3 using the Mo-Mn process which is a fairly involved technique but achieves very strong bond between the steel and ceramic. However, at 400F the differential in thermal exansion between Carbon Steel and Al2O3 might be enough to crack the coating. Are you stuck with Carbon Steel? There are other metal alloys that have thermal expansion coefficient that match closely with ceramics. One is named Kovar that matches with Al2O3.
Leonard Jesus is the WAY