Solution: Theme Week!
I'm just spitballing, but what if we have like a week-long theme where we propose a (prolific) tag and the goal is for participants to collaborate in cleaning up that tag? Cleanup goals could include (1) identifying duplicates (2) identifying "blind spots" where a canonical thread is required (3) answering neglected questions as well as anything else we think might be necessary. The idea is that because everyone is working towards the same, common goal in a focused way, participation will be more rewarding.
If the "teamwork" aspect isn't enough reward in itself, I think that some high-rep users (I'm including myself) could be persuaded to award bounties to users who make significant contributions to the effort. We'll clearly have to work out the details, but I think it is within the mechanisms of CV.SE and the spirit of the SE organization to reward these kinds of computation.
(This is a wholesale rewrite of my answer, which was more of a comment. Interested readers can find it in the edit history.)