Great idea. At Bountysource.com we have a plugin for GitHub that automatically updates issues with bounty details when they exist (adds a link to the bounty in the issue body, bounty total to issue title and a special label). This needs to be installed by a project committer though. I'd love to see a Bountysource Chrome/Firefox extension, maybe we should put a bounty out for one :P
This is exactly what https://www.bountysource.com/ does. Any number of people can contribute to the bounty pool. When a developer submits a solution that gets merged into the project and the issue itself is closed, that developer collects the bounty.