Sense, Nonsense, and Not-Sense

I'm sure you're right, it's too much work without a whole load of wrangling volunteers as AO3 has, with the userbase for AO3 much larger and able to support the effort.

A community with an index of communities is something I was thinking about as still needing quite a lot of work to keep up to date, but possibly the best way to have a single "Go Here" space to find communities :) It would need volunteers to check up on the links every so often I guess, and people could add their communities in tags to specific posts and have a volunteer/moderator add each verified community to the post proper and delete comments when it was done...certainly something to think about.
A machine generated index would be a good idea but you point out the privacy issue and a machine couldn't know something like I just joined, ushobwri, ie you should be writing, is a writing community (and one that requires you to join to see entries). The limits of technology!

It's definitely something we need, a way to connect, but the how is still a big question mark.