The rust forms pretty quickly there - hell, I see rust begin to form on the rotors in the morning if it rained overnight.

The rotor will get extremely hot due to the contant contact of the pads. You'll need a high temperature paint because of this. I would think automotive engine paint would be good place to start. A salesperson at an autoparts supply store can probnably point you in the right direction if unsure. Of course, don't spray them when hot.