Thanks only to having seen this same thing happen (vehicle backing up out of intersection) many, many times, IMO the best thing the biker could have reasonably done is anticipate this happening. I always leave at least full car length between me and the vehicle in front of me at an intersection just so this can happen. I expect it every time, even in my honkin' great work truck.
*edit- I leave that much space until the vehicle is stopped in the right spot. If he is over the stop line at all, I leave room anyway. I had a guy wait until the light turned green to start backing up. I think he shifted into reverse after he stopped but didn't need to back up, then forgot to shift again. No backup lights, of course.