Interesting, that is something that we had in mind in the past. I know there are few good websites doing that, starting with simple bounties like IssueHunt and ending with more complex models like Tidelift. I'm not sure how well those work, honestly.

One similar thing we were thinking of is giving maintainers the platform to easily provide paid support if they want to. Not sure about that one either, although I know some companies/people make money from providing support for OSS.