Hi Bagsmasher, Buzz here at PDXMM. Well, it is sort of a trap door, but only for Boeing. We have floorboards and seats in the way.
On rare occasions , very rare, engieering type people will have some new sytle boxes in the electronics bay forward of the forward cargo pit. They'll run wires out the hatch into the cabin if they have to. Yes, we can take out the wall where the electronics live. But then with all these bulkheads open, it's not smoke/fire proof. So the forward pit has to be empty, there's a hole in the floor that pasengers could trip on, things that give the FAA heartburn. So it's a "non-rev" flight which gives the bean counters heartburn.
But you can't do it if those seats, carpet, and floorboard are in the way. So it's possible, but only for engineering test hops. The 757 has something similar into the electronics bay. You still need a screw gun to get to it from the cabin. If you pulled up random floorboards like in one movie you'd have a hard time getting downstairs.