Since the recent move to storing playlists in the database, it's impossible to
hide music from the main library, but still have it accessible by a playlist.

I deliberately keep film soundtracks outside my MP3 library, but had playlists
in my playlist folder which still allowed me to play a soundtrack on demand.

I implemented this to stop film soundtracks from being added to the a shuffled
list of tracks.

I understand this is a strange usage model, but I cannot figure out any
work-around for achieving what was previously possible using file based playlists.

The rescan code does detect that the tracks are outside the library folder, but
still adds them to the Db, so perhaps a bit field could be added to the Db to
record the fact that these tracks are available for playback by playlist only.