you can lock directories, you can atomically swap directories (on linux), CoW filesystems make cloning kind of cheap. That could be used to implement transactions and commits. Getting the consistency checks/conflict detection during the commit right would be the most difficult part. Change notifications could be used to do some of that proactively. It's a terrible idea, but it could be done.

There is a transactional API for NTFS in Windows [1]. It allows transactional operations not just within a file but also across files or across multiple computers (to make sure something is applied to your whole fleet atomically).

1: https://en.wikipedia.org/wiki/Transactional_NTFS

Yup, the I in ACID is a bitch :)