Ha, it suddenly sounds like trouble.

Eg: one could piggy-back an entirely new file onto an existing file (it might have to be text encoded?).

It looks like the kernel might impose a limit of 64KiB on a file's metadata, but that's still quite a lot of room for data smuggling...

I've lost extended attributes of files on a number of systems because they aren't always included by default in file operations. I don't trust everything that could possibly write/update a file to preserve them--common 'safe' pattern is to rename as .bak and write new file contents without EA to original name. Nor do I trust myself from archiving without using the right options in a hurry.