Post History

At this early stage I think you'll simply have to give out the privilege manually to users per community, similar as to how we've gotten temporary moderators in place. Given that they are active users and interested in acting as "curators"/"moderator lite". Most communities have been around long enough to tell who's active at this point.

Also, if we trust a user with curate (or moderator) privileges in one community, there's no reason that they couldn't get the same privileges on other communities where they are also active. So maybe poke around while handing it out: "are there other communities you could do this for as well"? 

It's going to be impossible to hand this out automatically based on participation, since roughly half of our communities are doing OK but the other half are essentially "ghost towns". On the other hand, the "ghost towns" might not need any "curators" yet? We could start with the more active ones and see how it plays out there.