I agree, and on this application, space is limited, and safety becomes a factor. In most states if you add more seating capacity to a vehicle, it most have safety restraints. I have seen a shoulder harness added to a rear seat by tieing into the shoulder belt mount for the 2nd row and then mounting the belt roller to the floor and a buckle pigtail to the other side. But to me with the space restraint, I find it useless and out of place back there. Just my 2 cents worth.
If you are going to tackle it yourself, do it right, and not half ass. You may want to check into something like a jumper seat from a ext cab Ranger or a Dodge Dakota, I beleive the Dodges jumper seat mounts to the side of the cab, just like you need it without much modification, and with a jumper seat you can fold it up when not in use and have storage space restored when needed.