TL;DR:

Reviewers of a post should be able to comment on the post in a separate comment section only visible to the reviewers and 10k users and moderators.

Communication between reviewers of the same post would lead to better review choices.

Currently, the best option we have for this is the comments section on the post. These comments would unnecessarily flood the poster's (and any follower's) inbox. They might also be flagged by low-rep users who do not understand the concept of review, creating extra work for moderators. A non-reviewer might leave a review-like comment, potentially fooling the reviewers. Such comments might also be confusing to new users, and might even make them leave the site if a "this post is bad"-esque comment is posted. For all these reasons, "review comments" in the usual comment system are treated as abuse.

Such comments might also be buried under several other comments or be moved to chat, making it hard for a reviewer to see them. They might also bury other comments if other reviewers agree with and upvote the review comments, going against the purpose of a comment section.

Why would a review-related comment be flagged?

In the Suggested Edit queue, comments on a review task would not be related to the post itself, but to the edit. This would be a major reason for flagging. A "This post is okay because (something)" comment would also be flagged for being outdated when the post receives many upvotes. These flags would create work for mods.

How to solve this?

I am suggesting a feature request: comments on a review task. Any reviewer can comment on a review task, and such comments are only accessible to future reviewers who don't review the task as an audit and 10k users (and moderators). Such users can also post extra comments in the thread or flag or upvote comments. The same user can post multiple comments on the same task. All the comments on the review would be displayed somewhere next to the review task itself. Once a post is fully reviewed, its review comments are archived by the system. Just like normal comments, review comments should be upvote-able and flag-able by reviewers. Archived review comments would be visible to 10k users and moderators in a separate "review comment archive".

But wait: what about audits?

For that, I suggest that the system stores "audit comments" which are indistinguishable from review comments on a post for each specific type of review. An audit comment can be added by any user who completes the audit, just like with normal review comments. Audit comments from passing users are shown to the next user who is reviewing the post as an audit. Audit comments from failing users are archived, but not shown to anyone directly. Archived audit comments are visible to 10k users and moderators in the "review comment archive".

Why shouldn't other people see the review comments?

The review comments might be hostile to the post. If people who don't review the post and aren't experienced with review comments see those comments, they would flag for moderator attention when it is unnecessary, creating extra work for mods. If this section is visible to the poster, the hostility of this section on a slightly bad post might cause the poster to leave the site.

How would this help (example):

Imagine that a good answer looks like spam that got flagged as Not an Answer and sent to review. A new reviewer would just flag it as spam and leave it. An experienced reviewer would read the question first, then deem the answer as appropriate, but with the current system, there is no way for the good reviewer to tell the other reviewers other than leaving a comment (which might be flagged as "no longer needed" or "abusive" and deleted by a mod). More importantly, the comment might not be noticed, and a good answer might be auto-deleted as spam. Review comments would greatly help in such situations, since they would be prominently visible to the reviewers, and are not prone to flags by non-reviewers.

It can even catch robo-reviewers!

Robo-reviewers probably won't leave a review comment. Even if they do, it would probably be irrelevant. Therefore, if all review comments on a task that are posted by a user are flagged at least n times (choose your metrics), he probably robo-reviewed the task. This would make it easier to identify robo-reviewers.

Policies regarding review comments

• Review comments should be relevant to the review task in question.
• Review comments should not be used to say "This is obviously an audit" in any form.
• Review comments should be polite and not demand anyone to do something.
• Review comments should not say anything like "please upvote this post" or "please downvote this post" or "please close-vote this question" or "please reopen-vote this question" or "please delete-vote this post" without further explanation why.
• Most of all, review comments should follow the Code of Conduct and the "Be Nice" policy.

A moderator might go through your old archives of review comments and ban you from commenting on reviews (or even ban you from reviewing), so these rules are enforceable.