I know in WI you can without a seat or belts
Wisconsin Department of Motor Vehicles
May you ride in a pickup camper?
Yes, if the camper is attached, bolted, etc. to the truck.
May passengers ride in the back of a pickup truck if there's a topper on?
Yes, so long as the exhaust system extends beyond the body of the truck.
May you ride in the back of a pickup without a topper?
Yes, if you are over 16 years of age and seated in the box. It is illegal to sit on the sides. No child under the age of 16 may ride in the open cargo area of light trucks.
Here is a list of state laws, but it doesnt say anything about with a seat bolted in the bed.
In town you will have no prob (under 35MPH) but i dont know about with a seat if you could go faster or not.
http://www.iihs.org/laws/cargoAreas.aspx