Re: Nothing new @pPPPP re MP3 players.
Yes, but all you're doing is storing an index, in the same way that the permuted index for old-style UNIX man pages from 40 years ago allows you to identify pages that mentioned particular key words.
And if you break it down, in a UNIX-like system, the objects are actually tracked by inode which links blocks to objects (files), and the file tree structure is just a way of indexing the inodes.
It could be perfectly possible, if a little unwieldy, to have an index of inodes other than a hierarchy of directory indexes, but you would have to do something about permissions, as although the inode itself includes permissions that can be checked, UNIX also requires a permissions check on the path to a file, not just the file itself.
In fact, I understand that a number of POSIX compliant filesystem implementations do allow this type of access. GPFS (sorry, IBM Spectrum Storage, or whatever it's called this week) for example, has a policy engine that allows files to be accessed outside of the traditional file tree.