A few years ago I came up with the idea of an LED display for the rear window, with a minimal number of messages that could be displayed: "Sorry", "Please dip your lights", "Thank you", etc.
I never got around to doing anything about it, and some time after that I saw an advert for just such a device. I think the biggest problem would be that if it caught on, within a few days the net would be full of hacks to allow you to reprogram it with abusive messages. I'm afraid to say that I think most drivers enjoy being abusive, knowing that they are in a position to do it anonymously and with impunity.