Enforcing a contract is not done by writing it, it is done by taking the other party to court. Whether or not an income-sharing contract could be enforced is a legal question I am not qualified to answer, but I would be pessimistic. The contract might be nulled because it amounts to slavery, or the income-earner could declare bankruptcy, or so on.
Even assuming that you have the money for a lawyer, and the contract is enforceable, and you win a judgment, you'd find that winning a judgment is often a very different thing than collecting a judgment.
Many of these people could be earning 10x their living expenses
This premise seems questionable, though it's vague enough ("many" in relative or absolute terms? are we looking at wannabe researchers in some specific field or in general?) that it could be correct.
In general, I would expect that the amount of people capable of earning 10x their living expenses are quite rare, especially since a person tends to spend more money on their living expenses when they start earning more.
Depending on the field, research costs A LOT more than living expenses.
This is also overly complicated. There are many people who ALREADY earn significantly more than they need. It seems far simpler to convince them to fund researchers than it would be assemble groups of 10 researchers and hope one manages to strike it rich.
Finally, of the people who would like to do research, how many are actually useful? I imagine a lot more people want to do research than are actually good at it or working in fields where new research is crucial.
"Depending on the field, research costs A LOT more than living expenses."
True. I was thinking more along the lines of FHI or SIAI style research that doesn't need much if any equipment.
At least for the single, communal living with people on a shared project seems more likely to work. As someone else pointed out, enforceability, and trust, are obvious issues in your plan for "You support me in my research, and then when I'm making boatloads of money, I'll do the same for you."
This is vague enough that it might plausibly induce equally abstract solutions to the same problem, as it doesn't have many details to criticize. That would be good.
More likely, it won't begin a dialogue at any level of abstraction other than those below it. This is the problem with proposing solutions as a conversation starter.
In their minds, most people seem to have already chosen a solution to the implicit problem, and are concerned exclusively with filling in details (if they are concerned with the problem at all). In such situations, proposing a different solution seems like a fair opening.
There are more people who would like to do research than people that would like to fund them. Many of these people could be earning 10x their living expenses. Why don't they get together in groups of 10 with similar goals and draw lots to see which one of them gets a job, and that person funds the others?