This is a super late response, but your best bet would be to disassemble the wheel and get the rim and hub powder coated by themselves. The spokes have to be able to move somewhat for assembly and truing, and they're so thin that they'd be difficult to powder coat well.
There are, of course, spots on the hubs that you don't want powder coated, like the bores for the wheel bearings and the drum brake surface (if applicable to your rear hub). So keep that in mind.
