Post specific password protect comment entry

I’d like to make some posts globally viewable but in order to post a comment a post specific password must be entered.

Has anyone come across a plugin that does this or something similar? I’m looking at modifying the Password Protect Enhancement plugin but if something already exists to achieve this result I’d love to use that instead!

My thinking is that either using the integrated post password or a custom field to set the password would be the way to go. This is not just a captcha thing, I need a specific, unique per post password to be entered before the comment form can be displayed…

Thanks in advance!