You’re going to need to use pre_get_comments. Unfortunately, I cannot find much documentation on this. Nor can I find much information on WP_Comment_Query https://codex.wordpress.org/Class_Reference/WP_Comment_Query But it should allow you two alter the comments returned just like pre_get_posts. Here’s a general idea of what to do.
1) See if the user can see the secure area, if yes then return without doing anything
2) Get all the post ID’s for the posts in the secure area and add them to post__not_in
I don’t know if this will work.
