I'm really surprised that this sort of functionality isn't built into OS's/filesystems. I recently had to do this for HDFS, and I finally "gave up" and polled the file system like you suggest as your first option. Event notification seems like something that ought to be a fundamental feature and is best owned by the file system itself.
It appears to be built into macOS [1]?
> Whenever the filesystem is changed, the kernel passes notifications via the special device file /dev/fsevents to a userspace process called fseventsd
Which I assume is what they're referring to here:
> A platform filesystem watcher for Darwin is used, but certain event properties are handled by the standard library. Namely, the event time and the path type.
It is, but it's badly implemented and buggy. But the real problem is that there is no posix like specification for file system events so every platform does it differently. Even if every platform implementation were perfect and bug free, it is a huge pain to write wrappers for each one.
The magic file approach described by kevincox below is probably the best way to get > 95% of the benefit with < 1% of the work.