It could be done, but it's really not worth it. You wouldn't want to use the painted surface for braking, so you'd have to have the rims coated, and then grind off the coating on the braking surface. Anodizing is a lot stronger than powder coating and brake pads eventually wear away anodized braking surfaces. If you want a radical color, then powder coating would be fine, but anodizing would be better and there are plenty of places around here (LA'ish) that anodize aluminum parts. I've had some RC car/truck parts done before I developed a fetish for stripping and polishing AL.