Case-insensitive ext4

There's no point in special-casing "one-off opens" unless this demonstrably solves a problem. As then kernel open has to scan the directory, anyway, you'll end up with the exact same kind of TOCTOU races. This is a problem which can't really be solved. As to your other objections: These is a generic list of programming errors, some of them attributable to the idea with "a background thread".

It's possible to implement case-insensitive open in user space without doing a second linear search through a directory for every open.