They only care about commercial tags, in some states, if the vehicle is above a certain weight range. I can't speak for all of them, though.
You could just get some plain black numbers, put 'em on your front fender and on the back door when you need to look industrial-stealth without more signage. I see those all the time, especially on vehicles that don't get enough public exposure to bother with advertis-ey markings. Also, if asked by an authori-TAH where your commercial paperwork is... tell em whatever you like, but there's no more commercial requirement to have a number on your vehicle than anything else, like all those Airstreams out there with club numbers.