I think this is plugin territory. In my forum I have several threads where only admins can post updates for users. Not really a discussion topic, more of a change log or mini blog. If posts were to start merging the updates would get tangled.
Just one use case, but I'd prefer the core code not to automatically join double posts....or at least make it an option.
If you were interested in subcontrating the work to the nodebb team I'd be happy to look into it. I wouldn't mind giving you a discount on our rate for things that are on my todo list as well, since I'll be building it eventually. Email us at [email protected]
If not, I'd be happy to accept any pull requests to this repo and/or the cash plugin. Even if it isn't 100%, maybe if you start it I can help point you in the right direction