All the cars that I've had with folding rear seats -- Jetta, Saab/900, Volvo wagon, Subaru Legacy -- were set up that way. I think the problem is that if just the back folded down, you'd have a very high flat floor, or the folded seat would have to be higher than the trunk floor.

Maybe it would be doable in a FWD-only car with no tunnel (i.e. like a Citroen DS) -- the rear seat bottom could drop to the floor with the back folding over it.