Here's a quick'n'dirty userscript, tested on Greasemonkey on Firefox: https://github.com/BobVul/SEReopenReviewWarning/blob/master/SEReopenReviewWarning.user.js
Can be installed via the raw link: https://github.com/BobVul/SEReopenReviewWarning/raw/master/SEReopenReviewWarning.user.js
Currently only the pending/completed review warning is there. I might add the other bit (notice under the reopen?) later, but it looks to be more complicated.
It currently shows two possible messages under the close box: one for a completed reopen review and one for a pending (incomplete) reopen review.
Feel free to raise any issues or pull requests on the GitHub project, or comment here.