Thank you for the update. I passed this listing on with my endorsement to several people in the DC community who applied and heard nothing back. Having the event fall through like this is discouraging.
In the future I’d urge you to share bad news sooner, it doesn’t get better with time.
Be very careful about introducing an expectation to pay. Payment, even contingent future payment, will fundamentally change the nature of the endeavor. People are very motivated by social norms in most situations, but introducing money buys your way out of those norms. The classic treatment of this is is that daycare centers that fine parents for tardiness and making the staff stay late have much more tardiness than those who just disapprove.
See https://www.nytimes.com/2005/05/15/books/chapters/freakonomics.html
To the extent that this practice stays within a community, the community’s own norms are probably sufficient. Besides, many/most hosts would probably prefer guests donate to charity if they end up well-off. Maybe you have some sort of soft norm that guests offer to let their prior hosts direct some of their altruism, but be careful even with that. Everything matters on the margin, and that’s about the upper limit of incentive that won’t meaningfully displace the group norms.