Please consider a "Hide post" function to the Admin/moderator tool set. Not to be confused with unapproved which will go into the moderation queue or delete post which hard removes the post from the database or private which can only be applied to topics and not posts.
Hide post should do just that, hide the post from everybody except mods and admins. Perhaps also add an indicator on the post that tells the mods/admins that it is a hidden post
We have this..... click the eye on the first post to make the thread private. Then it can only be seen by mods and admin and the person who wrote the post.
Anything else can be done by making a forum section that can only be seen by mods and admin and you can post there too 🙂
@verek oh you want make individual posts in a thread private? my bad 🙂