have a list of rewardable issues. grow the reward for each issue with time. once the reward makes the time spent on the project worthwhile, people will be incentivized to commit. they won't let the reward grow too high, for fear that someone else will take it.

you could let coders freeze the reward for some amount of time, and get exclusive rights to claim that reward, until the time expires.

[1] http://en.wikipedia.org/wiki/Moving-knife_procedure