Working with UTF-8 in the kernel

Nothing scary about that: Open directory (or use an already open descriptor), acquire lock which prevents adding/ removing entries, process accumulated change notifications, create/ remove entry, unlock.

Such a lock must already exists, BTW, it may be sufficient to expose that. Advisory locking would probably be ok as UNIX processes are usually supposed to cooperate and not fight with each other. Vastly simpler in the kernel than 'hard-coding' a specific, known-to-be-broken/ deficient case translation mechanism into certain filesystems. Considering cases like "vertical bar plus combining overline' (aka T, not going happen as that's an ASCII codepoint), I consider "kernel supports UTF-8" much more 'scary'.