Restrict users to comment on specific posts

I just received a request from one of my clients. Here’s the a breakdown of the request.

There will be 10 authors who submit 1 document for review. Each author will have their own page/post that contains the link to the document with the ability for a specific set of users (20-24 users total) to provide feedback on the documents via the comment form.

Here’s the catch: They would like all users (10 authors + 24 users) to be able to view all documents/comments, however, users could only comment on the pages they’ve been assigned to.

My recommendation was to use the comment system to approve comments from users “allowed” to comment on each post.

However, we were both interested in seeing if there’s a way to extend WordPress in a way that you can restrict users from posting on specific posts.

Any ideas?