I think it's safe to say that it is possible.
but a lot more complicated about standards and regulations. Would it be properly engineered? properly anchored to structure? etc...
I don't know where you'd find out for sure. Talk with an automotive or RV engineer I suppose and get pointed to the chapter and verse in some obscure list of codes. Hopefully someone like that will chime in here....but in all the discussions about seat belts, car seats, and such I've never seen a definitive source chime in with that sort of info.... on this or other forums either.
But think about this....how many times have you ridden in a charter bus, rental car bus, etc... without seat belts. Just the other day we were riding the street car trolley in downtown Tampa, and I was explaining to my kids what the handles hanging form the ceiling are. They let you stand in things like that!
so
don't you think that pretty much anything you could come up with would probably be good enough?
maybe not good enough to meet child seat requirements, but for an adult to sit in.... the back of an open pickup is fine...