I think this is a very good idea. We have already been discussing adding a step in the interview process where candidates would be paid to submit (and merge) just such a type of issue. But the problem with the during-the-interview approach would be that we’d have to give each candidate their own bug.
If, on the other hand, we have a list of open bounties and some people work on them simultaneously, whoever gets the fix merged first gets paid. 
I’m not sure we’d want to restrict the pool of candidates to those that got bounties, though. But maybe we could do both simultaneously, until a time when the bounty list is popular enough to generate enough candidates for our hiring needs.
(Cross reference to the help mailing list)