I have those same (or very similar) trailer lights that Christ posted on my car trailer. They have two bolts in the back, making them very easy to mount. Another bonus is that these lights are all LED. The white lights that are designed to point downward from the corners of a trailer are arrays of maybe 6 LEDs that point all in slightly different directions. This feature would easily lend itself to adequately illuminating a license plate, however you'd need to modify the housing because I think you need to have the directional blinkers pointing sideways. IIRC, the stuff inside the housing is pretty movable and it's easy to get in to. Nice lights.
Edit: I couldn't find anything explicit about rear-side direcitonal visibility, just rear directionals.
http://public.leginfo.state.ny.us/me...MMONQUERY=LAWS