What would happen if you stenciled paintings of license-plate-like patterns all over the back of your car? Then you're not tampering with the plate itself, but I guess you end up with a goofy-lookin' car.
>>troyvi+(OP)
Why do you think tons of fleets have 6-char alphanumeric vehicle IDs?
Gotta have some number on the thing for your own ID purposes and if it saves tou a $50 semi truck toll bill 1/500th of the time that's icing on the cake.