Mattias:
Are you asking if I've tried this:
(dir-locals-set-class-variables 'auto-revert-with-polling
'((nil . ((auto-revert-use-notify . nil)))))
(dir-locals-set-directory-class
"/tmp/" 'auto-revert-with-polling)
No, I haven't.
Which version of Emacs would you like me to try this with? And what result are you expecting/hoping to see? (e.g., might it reduce the current 30-80% CPU load for polling with emacs 28.0.50?) There are only a few open files directly under /tmp, so would this have any effect or does it propagate down to subdirectories?
[Also, I'd need a few more details (not being an emacs-internals person) ... should I add this to my .emacs and restart, or execute in a scratch buffer, or ...?]