can root somehow create a immutable file in a non-uid-0 hierarchy?

[Skunk Worx]

fc6 user...basic fs question.

Suppose I am root/UID 0. Is there a way to create a file in someone 
else's (non-uid-0) FS hierarchy and have it be immutable? No rename, etc.?

I'm trying to do a hot-plug of disk keys and I would like to put some of 
my own bytes somewhere so I know whether or not I've seen the disk key 
before.

Some years ago I used SGI machines and they had a file system with a 
volume header I could use. I don't think extX has such a capability.

I'm pretty sure the answer is no, but I figured I'd ask anyway.

Thanks,
John