Plugin to comment/collaborate on backend

I’ve got a client we’re creating a news website for. They want to have their entire editorial process including communication around articles happen within the backend of the website.

I’ve been able to meet all requirements except for the ability for the authors, editors, managers to be able to communicate with each other, on the edit screen of the actual article (post).

To be clear, they don’t want the ability to comment with readers from the backend, they want to have internal dialog throughout the process of writing an article on the actual article and then hit publish.

Here’s an example use-case:

1) Author creates new article
2) Author writes article
3) Author saves the draft
4) On the same edit screen the author writes a comment asking the editor to look at it
5) The editor takes a look and comments back (same screen) saying, “yeah this looks good but change one thing”
6) Author changes one thing, then comments back
7) Editor says it’s good
8) Editor hits publish
9) Anyone can view the page and see the entire comment history around that article

I’ve already mentioned to the client tools like Basecamp and Slack that are much better suited towards this type of thing, but they really want to keep everything they do inside WP.

So what I’m looking for here is a plugin–if it exists–that can handle something like this. And if something doesn’t exist then I’ll need to build it for them.

Not posting on StackOverflow or WordPress.StackOverflow due to the opinion nature this question would generate.

Thanks in advance