NTFS can have additional data streams associated with a file. It's used for stuff like the warnings you get when you try to run an executable downloaded from the Internet. This seems like a perfect solution for me, at least on Windows.
Think of things like any of the cloud drives, bittorrent, screen-sharing or IM software. Are they aware of ADS (or equivalent for that platform). What would the right thing to do be when syncing between platforms that have support, and ones that do not?
However file metadata is usually pretty poorly understood by most developers (based on a personal sample of my past experiences) and file system resource forks are even less understood...
Perfect is the enemy of good and all that sort of thing ...
Outside of Office sometimes trying to encourage using it to make searching easier and Windows sometimes using it for ID3 tags from audio files, I've never really seen anyone try to use it.