No laws against it that I'm aware of, as a matter of fact there were little trucks made with seats in the back (out in the open) back in the 80's.
A blazer with a removable roof isn't any different. Take the roof off and like magic, you have a back seat in the bed
A seat with a lap belt would be much safer than just riding in the back of a pickup and sitting on the floor, and I see people doing that all the time, even I've done it. I've never once been questioned about it. The places I've lived don't even require rear seat passengers to even wear a seatbelt...
As far as the tailgate up or down law, that sounds like a load of bull, I'd doubt that it could be enforced unless you had a load of something that wasn't tied down properly. Otherwise it doesn't matter whether it's up or down or even completely removed.