Anything that is e- conductive can be powder coated. Even Carbon Fiber materials. I have even heard that some plastic parts have had small amounts of metal/conductive materials mixed in with them, so that the part could later be powder coated.
the only thing you cant powder coat is anything that will melt. Remember you have to put it in an oven at a pretty high temp to cure the powder. I poweder coat all kinds of different metal at my work so stainless should be a snap