What about going the other direction and doing really thin seat, that could bolt to the top of your drawers? Like an early model FJ40 seat... you'd need to get a little creative with the seatbels but that would be pretty easy for lap belts. Then you'd have actual drawers under them, and the kids could sit on top.
Another idea is on my GX I built a 1/2 width cubby/storage on one side, so that the other side 3rd row could still be used. Not sure if you thought of that one yet...
My experience with all of the drawer systems I've had/installed is that the install is just too difficult to want to do it more more than once, and it's just not practical to remove them to use the seats again, plus they take up a ton of room so you'd need space in the garage or shed for them when they're not in the truck.