I have now built this functionality.
There is a new site setting private_replies_participants_can_see_all. Once that is enabled, replies will be visible to users that have replied to that topic.
I’ve also enhanced the banner that shows on top of the topics to be more accurate in communicating who can see what.
If you’re hosted with Communiteq and are using this plugin, you’ll get this functionality in the next few days. If you’re self-hosting, just rebuild.