This just came up again for me because I put a lot of work into making playlists from some cds that friends gave me. Making the file read-only DOES seem to work, although it's kludgy. MusicBee throws an error if you try to reorder tracks, and if one is deleted the entry is still there, just with the path of the missing file. Those are the things I care about, preserving playlist size/order and a minimal amount of data about missing tracks. I'll have to see if I can reconstruct some of the old playlists I lost through carelessness, now.