wpDiscuz Private Comments addon is an easy tool to allow commenters create private comment threads. These threads will be available for website administrators, moderators and for the thread owner. Addon adds a small button under comment textarea to set it private, also it adds [Private] button to parent comments to make it private after comment is submitted. The buttons to set comments can be displayed / hidden for certain user roles in dashboard. Also it allows to set certain user role to moderate private comments. This feature can be enabled / disabled for certain Comment Form in case you have many custom comment forms for custom post types.
wpDiscuz Private Comments addon Features