If general consumers of OSS act entitled, then how entitled are bounty contributors who think their issues haven’t been properly fixed to their satisfaction going to be?
A person who wants to pay for a bounty would need to submit an issue, and the maintainer estimate it's complexity. At that point, the person filing the issue could click "Buy this issue" if the maintainer has bandwidth.
Again, not easy -- and I'm sure the UX / business / execution would be delicate.