Hi Sassy,
Each state will have their own regulations and those are probably found under the DMV. For example...you build a sandrail (dunebuggy) you want to license for the road. Out here, it has to have a basic inspection for required equipment (seatbelts, lights, brakes, horn, etc). These machines often use a simple padded, fiberglass bucket seat. The last one I registered the state cop checked the mounting bolts for both seats, as well as seatbelt mounts.
Since we have no annual vehicle safety inspection, changing the passenger seat in a van might never be noticed, but I'd hate to argue the point with a cop on the side of the road.
If I were to do the office chair trick, I'd drape it with a colorful Native or Mexican blanket that hides the base. I just don't think I'd feel comfortable with a passenger using it safely.
I'm sorry I don't have any hard facts to offer.
Oh, if I were in a van and did this, the footwell would be converted to a vented battery compartment most likely.
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