I'd like very much that members have the opportunity to flag a thread as being relevant to one or more secondary usergroups. members of that secondary usergroup could then view all of these flagged threads in one virtual subforum, even though the threads in question were spread out amongst the entire forum.

It's a bit like a subscription list except, you don't have to find the threads and subscribe to them. They merely become visible to you and appear to be assembled in one place because you are a member of a relevant usergroup.

I'm not sure how the threads would appear. Ideally, a different forum home page would be presented to specific usergroups in which their virtual forums would be presented, say, at the bottom of the page.

I foresee that someone starting a thread would tick a box indicating that the thread was relevant to secondary usergroup. They could select the secondary usergroup from a drop-down list. After that the thread would be allocated to the virtual subforum and could only be unflagged by a moderator with sufficient privileges.
We could make a contribution for this work.