Just had the same problem in my GT after folding the seats fully flat (base cushion tilted forward then backrest down)
Thanks to the posts here I was able so solve it relatively quickly. It seems that the belt will lock as soon as the seat tips a few degrees forwards, so mine wasn't actually "stuck" it was just "locked" without enough slack to tip it back upright.
I undid the bolt holding the static end of the seatbelt (on the floor) by holding up the rear cushion and squeezing in underneath with a socket wrench. There is a plastic stopper on the belt which prevents it from retracting any further so there should always be just enough slack to get under and undo the bolt. What had happened in my case is the plastic stopper had slipped through the plastic slit on top of the seat and gone right up to the belt reel, but I could still lift the seat Just enough.
Once undone you can put the seat fully back and, true to the posts above, the belt should release. I think this is more due to the seat being totally upright and the lock releasing but I pulled upwards anyway for good measure.
You can remove the cover from the back of the seat but all you gain is access to a couple of inches of the belt as it enters the reel, as the reel mechanism is behind a metal plate.
Next time I put the seats down I will pull a little slack through the belt and clip it into the farthest side to keep some extra slack in case it locks again!
...next job Door handle :/