I have a 2-seater middle seat, tan velour/end armrests removed mounted *backwards*, similsr to the single jumpseat, against the front seats in Cunegonde the '83 Westy. I made steel reinforcing plates with captive nuts, siliconed on the underside of the floor... Installed while fuel  and camper LP tanks were out. 

The seatbelt shoulder harnesses share bolts with the front seats, but our middle seat is not fitted for headrests. 

We occasionally use Cunegonde as a travelling office, and the rear table fits nicely in between the back bench amd this additional rear-facing seat. One can still access the fridge (but not the silverware drawer or lower right cabinet), cook, and even extend the rear bed. One loses the walk-through, of course.

Speaking of the walk through - is there a center seat insert or front bench available for the Vanagon? I thought I saw something in a NATO Doka... Sometimes we want to carry 7 in the Westy.