I saw a feature in a car mag. a while back, on a massively modified RS4 (something like 800bhp). So desperate to get enough cooling to the engine - all the characters of the registration plate were cut out, obviously still looked black as there was simply nothing there, just looking through to the darkness of the engine bay.
Be it one of these, or the knida thing your mate is after - can't imagine they'd be legal, but novel.