You can't inherently search on it (i.e the filesystem doesn't manage its own index) but it would be trivial (with something like inotify or its successors) to w

You can't inherently search on it (i.e the filesystem doesn't manage its own index) but it would be trivial (with something like inotify or its successors) to write an app to monitor the filesystem for metadata changes and index them itself. This is how Spotlight works on OS X, with HFS+'s arbitrary metadata - the spotlight daemon watches for FS events and updates its index when a change occurs. I strongly suspect that this is actually what was happening in BeOS as well, at least on some level. It would be surprising to me if the filesystem itself was also what maintained the index; it seems like this would reduce write throughput if it had to update the indexes all the time as well.