Just thinking out loud here.

Could this issue be solved if the front-ends facilitate expanded use of community tags?

The front-end’s edit module could list all communities (e.g. in descending order of engagement, with title), then allow authors to checkbox pick up to 10 community tags, in addition to the 10 standard tags.

So, each post could have up to 20 total tags, 10 topic related and 10 community tags. Or whatever numbers are deemed appropriate.

Then it would be up to the individual communities to police abuse of their individual tags.

Would this work, or am I missing something?