You know the Israeli day care center study, where parents who were fined for picking their kids up later were more inclined to be late, because they perceived the fine as a fee-for-service? I think that formalizing the overweight-versus-space-donor relationship would lead to even more antagonism, on the same theory: "What are you looking it? You're getting paid good money to be sat on, shut up, I don't need to cooperate with you."
I do know that study. The problem that study found is that the price they put on being late was below the social stigma.
The solution is to make the price above the social stigma.
I think a reasonable rate would be $100 fines, to be handed out no more often than once every 10 minutes, half of which goes to the afflicted seatmate. If you are in the space of seatmates on both sides, you pay the fine once per afflicted seat mate. If you need to move to avoid continued paying of the fine, then you owe the airline for a second seat.
This is on top of the social stigma of having had people come, and explain how you're breaking the rules in front of the rest of the plane.
At that rate, a person who can't fit in a normal seat will see buying 2 seats for their cross-country flight as the cheap option. And one who isn't comfortable sitting in one may think extra hard about remaining uncomfortable rather than inconveniencing someone else.
