Recently I had the torsion springs replaced on my garage door.to determine the spring tension the door had to be weighed. They put two bathroom scales down , one near each end and lowered the door and recorded the weights and added them.
I thought about putting 2 scales down and make a plywood platform with cut outs for the weight displays. Assistance would be needed but it seems doable.
You would have to subtract the weight of the wood and your wheelchair of course.