Hmm.....how about the best of both worlds, the durability of stainless with close to the weight of aluminum......make the plates out of regular steel, maybe a hair thicker than you were going with the stainless, and then the customer can paint or powdercoat as wanted. Personally, I'd get regular steel and POR-15 it. That stuff is nearly bulletproof. The only drawback is that it doesn't stand up well to lots of direct sun exposure, but as a skidplate that's not a problem.
Not to mention steel is a LOT cheaper than stainless or aluminum.