Implementing fully immutable files

Searching lead me to one interesting use case - marking mount point directories immutable to prevent accidental creation of files when the filesystem isn't mounted there. It seems a bit niche though.

As for security guarantees, wouldn't something like SElinux be more appropriate, fine grained and auditable than this mechanism? That said, I have no idea if SElinux or similar behave sanely if policy is changed while files are already opened or memory mapped...