I like the idea, though I think to be more effective I'd prefer a brighter, wider green line. Of course, then you'd have to use more batteries.... Also unless the rear light happens to be the best on the market, I'd also think you could be better off having this as just the parallel light lines -or even just one off side light line -rather than integrating it with a rear red light. Let the rider decide which red rear light they want, and let this device concentrate on providing better, brighter and wider green light lines when the rider decides to turn it on.
But I think it's a pretty neat idea -and just one more thing that can be used to hopefully achieve better safety. Assuming reasonable cost, I'd buy one. I'd probably be more interested if it was just a separate device as I mentioned above and with brighter wider light lines too.
If it only works just once when you are out riding.....