I agree that having a bounty system would not work well and would only create division and hiring a dev for only a short time to work on something the community is already working on would also not work well as by the time they are up to speed of the code base their time would be up. The only thing here that could work is every X amount of months(say 6) a major improvement or feature is identified and under normal conditions this work could not be achieved by the current dev team. This work would have a set budget and terms and conditions.
If no major bit of work is identified that cannot be done with the current dev team then the money can be saved until then.
So in short if you are exploring the idea of hiring a dev the work that they would do would be significant, out of the skill level of the current dev team and the outcome would either lead to better tools for the devs or a big improvement to the game.