@[email protected], with regard to your idea about pulling in topics by mentioning the group actor... I don't think it need be as complicated as that.
A simpler UX flow would be for a user to "cross-post" a topic to a local category. In fact NodeBB already allows for this, I regularly move microblog content into local categories for discussion purposes.
Now, granted, it's "moving" a topic, which isn't quite cross posting, but I'm hoping to distil the idea down to some actionable UX and activities so it can be done in a common manner across different implementations.