awesome mod for hauling stuff.....the kids use the pass through all the time to hide theyre toys...

The seatback doesn't sit lower than the bottom cushion. Remember you have to tuck the bottom under the seatback when re-installing the bottom?
I think it'* fairly doable, but you will need to make sure you have a good latch setup. You'd also need to make a good backing plate for the seat as the foam isn't very sturdy -a good tug when you thought it was unlatched would rip the screws right out of the cushion.
For the latch, you could just cut a notch in the rear dash cover and bolt it to the top of the dash and do a cutout in the top of the seatback where'd you install the part of the latch that is on the seat. Would also be cool to convert the seat to a 60/40 split so you could just have one part open if you have 2 passengers.
This project is VERY doable and I bet it could be done in one piece form for under $100 dollars and if you were to make it a 60/40 split back it'd probably be another $50-100 to the upholstery shop and some pocket change for the aligning components (to keep the bottom of the seat backs even -possibly another hinge at the inner side of the seat) from a car in the salvage yard.