Random numbers right now. Luckily my state allows a year matching plate for display purposes. I'm thinking of getting a reproduction 71 plate, with a custom message, then painting it to match the tail panel (black).
The statute doesn't specify color or reflectivity or anything. Since the vintage plate doesn't have the numbers registered to me anyway, it might as well be total gibberish, or not be there at all!
You still run the risk of being pulled over and asked to produce the real plate anyway. Copy of the statute wrapped around the actual registered plate would do it I think.