I’m not confident that I fully understand the use case, but my take is that it is for post level read permissions within a topic.
IMHO, tangential discussion would be best in a PM.
If that discussion needs Moderator participation, the Flag “something else” will work.
I imagine that most Moderators getting a Notification about a topic they are not Watching and have nothing to contribute to the Flag message would not find the Notification for it to be that much of a problem.
On the other hand, a post stream within a topic that displays differently for different users is prone to presenting a confusing fragmented reading experience.