Currently, actors that a tag or category actor is following can post new topics in that tag or category.
If you have a category in your Discourse called “fediverse”, and fediverse@yourforum.com is following an account on Mastodon, say fediverse@mastodon.com, then every time that fediverse@mastodon.com creates a new toot, not in reply to another toot, that toot will become a new topic in your “fediverse” category, and any replies to that toot will become replies in that topic.
Applying that to your case, if the “I” in that sentence is a fediverse account that TAG@my.garden or CATEGORY@my.garden is following then when you either create a new toot, or boost a toot (aka “announce” it), it will become a new topic with TAG or in CATEGORY.
We may add the ability to push content sent to a tag or category actor by an external actor not followed by the tag or category into the review queue. That would be relatively straightforward implementation wise, but filtering would be a significant issue. Reviewing all of the incoming content would not be feasible in quite a few cases.
For example socialhub.activitypub.rocks has twelve Discourse ActivityPub actors (tags and categories). It gets (literally) hundreds of incoming POSTs every minute. If you deployed a review system like that there you would immediately blow up the review queue, making it unusable.
This has some potential as an idea, partly because you can already do this, check out “ActivityPub” in your profile and you’ll see you can link actors on any external Discourse or Mastodon with your Discourse user account. So one way of filtering the incoming of non-followed actors could be content from actors linked to users with sufficient trust on the forum.