I had the same problem. I managed to release it by removing the pin that holds the belt anchor/ seat hook assembly, and releasing the belt anchor. Then I raised the seat to the upright position, pulled the belt out from the seat and secured it with a clamp. Then I folded the seat again and replaced the belt anchor and reassembled the pin. The belt is still being held by the clamp until I can get a replacement cover for the top of the seat, which is broken.
Ihatemycar
March 2013