Jeep bench seat solution....
I have been looking into the very same thing for a couple of weeks now. Here is what We have figured out (in Canada).
Jeep bench seats are under 3' wide and less than 2' deep. The whole assemble can be purchased as one complete unit. The jeep seat has the kids sitting about 10 inches off the floor (facing the back) which allowed my 11 year olds to sit with a bit of head room (but not much).
The big problem seems to be the space from the back of the middle row to the inside of the rear door. We don't know how the seat would accomodate their legs yet. (could be a big problem) Today we are going for, yet another, test drive and this time will be taking the FJ down to the "Jeep Doctor" in Winnipeg to see if he has a seat that we can actually put in for a fit test.
In Canada, Transport Canada seems to allow this type of modification as long as you follow a long list of requirements. I believe the regulation is # 209 and covers things like how it is to be secured etc etc.
Hoping to make it work but also trying not to get my hopes up.
Good luck.