I've got a PIE script that puts a quick message on the screen every time a race restarts, just to warn noobs and remind regulars. (you can vote to restart a race on PiranMOTODestructionDerby to see it for yourself)
It's just waiting for a race restart, putting a message on the screen, and setting a callback to delete the message after 10 seconds, but you can tweak the timings. It's not quite on the green light, but it is right before it.