It's not an easy problem to solve. I could see there being a queue of bounty slots available for purchase that maintainers set and could be increased or decreas

It's not an easy problem to solve. I could see there being a queue of bounty slots available for purchase that maintainers set and could be increased or decreased at anytime. 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.