Hey suff@piefed.social let me share what I've been working on here at ActivityPub.Space
We're running NodeBB, which is a federated forum software that interacts with the threadiverse/forumverse natively, but also interacts with microblogs.
Anything coming from microblogs has no community attached, so we throw it all into a single "uncategorized" bucket.
This forum I am posting from is about ActivityPub, so I set up an auto-categorization rule so that posts using #ActivityPub get slotted into general — sounds good, right? Except that tag has a fairly low SNR. Every day I have to remove 4 or 5 posts that are at most tangential to ActivityPub at all.
I ran into similar issues with #climbing for a rock climbing forum. People use #climbing when they climb rocks... but also trees, fences, buildings, etc...
So it can work, it just needs some ongoing curation. Essentially what I am trying to say is... hashtags are fine, but its usage is nowhere near consistent enough to use in a global context like you are saying.