Been done before, I'm pretty sure. You have to replace the rear interior panels.
What I did on my old car, is to unplug the motors, using the small access panel where a rear passenger's knee would be on the side, to get to the motor. Unplug it, and then if it's stuck forward, use the lighter colored knob to turn the top portion of the belt back. It will stay that way with no problems. The only thing is, you'll either have to pull your belt over your head, or pull it up from the side and click it into place every time. On both seats, if you have the chest belt unlatched while the car is running, the light and dinger will come on. So you'll either have to remove the seatbelt light and dinger if you're driving while not using the chest belt on either side, or you'll have to do some rewiring with the seatbelt wires.