Well, my family (3 kids) used just the lap belt in the center seat all growing up... but we were never in an accident.
There are alot of resources for seat belt safety, just be sure it is from a reputable source (car manufacturers, government agencies, etc.), not groups that profit by scaring you.
The fact is that the center seat is not a real safe place to be in a high speed crash with just a lap belt. There may be some retrofit kit out there, but likely not for the 4runner as there is no anchor point for the shoulder strap (the seat back is not an anchor point). If you find one, I would scrutinize it heavily to make sure it is actually connecting to secure anchor points.
You might look into rear facing seats you could add. I am not sure of the safety of them, and a quick search didn't turn anything up. I know there are a couple threads on here about them for 4runner. They aren't going to be comfortable for a big kid or an adult.
Anyone know about the safety of a rear facing back seat?