Sounds possible. However, I would make it optional. Some people might
dislike this feature, invisible under the hood. And the timeout of 100ms
might vary depending on the filesystem the supervised file is located
on. In case of remote files, this will be too short I fear.

I will put it in my TODO. If time permits, I'll work on this, if nobody
beats me. Saying this, it might be better if you send this proposal via
"M-x report-emacs-bug". I will convert this into a wishlist item, and it
is much more likely to be found by other people than on my private TODO
list.

Best regards, Michael.