I guess the real problem here is that we should be able to use the aptly named 'newsgroups' header field -- just like USENET in the days of yore. Otherwise there's a problem aggregating/routing replies. Especially with Mastodon involved, because the authors of each comment can change the recipients to anybody they want.  I brought up a 'replyTo' field for this use case a few years back, but it didn't get any traction and was subsumed by Conversation Containers. I guess the bottom line is that I don't see any way out of this without an FEP; because each commenter will need to be able to automatically route replies to every forum involved. Best I can come up with on short notice is Conversation Containers where the context and/or target are arrays of Collections and you'll be sending your replies to all of them.

Might want to use a custom taxonomy/tag type for cross-posts. That's what we use for saving messages in "folders".  While we don't typically federate these, I would imagine  you might want to share those tags with software that supports cross-posting.