After several days and no response from anyone on this topic, have you considered just pulling your wheels, hit them with a light sanding (inside and outside the rim), swab the sanded areas completely off with some Xylol on a clean rag, then coat your rims with about three coats of "Maco in a can" spray in the color you want. You can leave it at that or you can spray some clear coat over the final color spray. But if you intend to beat the crap out of your rims in rocks I don't think I'd mess with the clear coat. Once you have them scuffed up, you can easily hit the scuffed areas with light sand paper again, swipe with Zylol, then touch them up to like new with the "Maco in a can" spray.
If you're hell bent on powder coating, no doubt it can be done by some shop somewhere. You might jump to another forum with your question and maybe hit pay dirt with someone who has done it and can provide you some info on the range of costs to have it done. They could probably also tell you whether it was worth the expense. Personally, I have never been impressed with the powder coating process. Any steel in proximity to being peppered with rocks and exposed to road salt will eventually begin rusting. Powder coat is probably superior to regular spray, but good quality can spray with good surface prep competes with appearance is less expensive, and you can keep them looking good with maitenance..
