Plaster wont survive the heat needed to powdercoat something, so thats out. The temperature to clay-fire something is very close, if not slightly higher than the temperature needed for coating, but I dont think the duration is long enough. I guess the best COA is to use high-quality enamels or acrylics with an enamal clearcoat mixed with doping compound. That way, even the salt in your sweat wont effect it, and touchups will be qick and easy.
I think the best material to use for day-to-day wear would be either aluminum alloy (like my dogtags) stainless steel, high nickel steel or 4130 chromemolly steel. They are all really strong and could be painted amount of times, and the steels would easily withstand the powdercoat process.
-Zombie