Can I legally transport someone in a wheelchair in the back of a minivan without a seatbelt?

There is an existing shoulder belt where the back seat normally goes. I am wondering if I can just make use of that somehow and use some tiedowns for the wheelchair itself. Basically I want to make my own tie down system so that I do not have to do any real modifications to the vehicle. (The vehicle belongs to a friend. - It's not my own.)