UID/GID identity and filesystems

Why not just embed an /etc/passwd-alike list in some xattr on the root directory of each filesystem? The host OS is free to mangle said list as it sees fit for security reasons. This wouldn't just solve the common irritation of off-by-one UIDs on USB disks, it'd make things like sshfs heaps more usable.

There's already popular prior art in removable media filesystems for storing things in a username-centric way: tar.

The entry price may be a little steep, but for that kind of usability, even I could be convinced to part with 20 or so bytes of my disk.