Sure, the U12 + U13 chassis' may be very similar (Hell, one's an evolvement of the other), and as such yes the bar might go on without any major dramas, but I still think mixing panels / bumpers would look very awkward.
Sure some commodore drivers put newer model bumpers on their older cars which looks OK, S13 / S14 / S15 do whole conversions on the front/rear ends to create a half-half mix, but the lines and shape of the U12 and the U13 differ so greatly, I don't think it would be possible to pull it off with it looking clean and neat.