I have been thinking about this for years. Given today's safety features and computerization I don't know if this is possible, but I think the least complicated front-back conversion would be a removable seat back. The sitting portion of the seat remains stationary. In a perfect world, if the seat back had posts and post holes, you could disengage some sort of cotter pin, pull the back out of the rear post holes, and then reinsert it into front post holes. It would definitely be a custom seat in order to make it both possible and comfortable to sit facing either way, and I'm pretty sure it wouldn't be legal for the driver's seat.
My old van has a jackknife sofa - basically it's a special hinge that allows the back of the sofa to do a half roll and become part of the mattress instead. I wonder if a hinge could be designed to do a similar thing, similar to what the person talking about the trains said.
Another alternative if you don't use the passenger seat is to simply remove it and replace it with any custom sort of seat/container/table combo you can design. This obviously limits this to a being a one-person vehicle, but it could open all sorts of possibilities for better space utilization.
