I have been trying to think of a way to store my playlists completely independently from any one music player, and I think that one way to do this would be to use each song's tags to store the list of playlists in which that song belongs. For example, if song1.mp3 and song2.flac both belong in the "Cool Songs" playlist, then I would add to each of them a tag called "Playlist" with a value of "Cool Songs".
To access the "Cool Songs" playlist in my music player, I simply search for songs where the "Playlist" tag has a value of "Cool Songs". Obviously, I would need a music player that allows me to manipulate arbitrary tags on any music format, including multiple instances of the same tag (so that songs can be in multiple playlists). Instead of creating playlists, I create "saved searches" or whatever the music player calls them, that search for the appropriate playlist tag.
Is this scheme possible, and how many music player programs would support such a scheme?