Forget about vanity license plates. Get yourself a vanity car! How about this? It would work best on a car that has the license plate pretty flush to the back of the car, no deep recession, and if the color is the same color as the background of the license plate. For example, you have a white license plate with black lettering, and a white car. Then you could carefully paint in the same font and size as the letters of the vanity plate, but with a whole phrase, where the license is just part of it, like:
with "I'd rather b" painted to the left of the the plate, and "g the piano." on the right, and the plate itself saying E PLAYIN. To make it maximally cool, the plate itself shouldn't make sense. Please send a photo if you do this!