Cool idea! Definitely an incentive model worth considering.
Effectively the nodes validate that the upgrade is worthwhile, not the bounty, but that does mean that hard forks are possible if different nodes use different versions of code. But its a good idea for a validation mechanism.
Question would be who decides how much funds are distributed for each upgrade? Could any developer be rewarded or only a select few? Who stores the funds? Its interesting to design an incentive programme with no centralisation risks.