Have a skin with a hole in it.
Make a playlist that fits there.
Have a "lock playlist" button on the skin, which puts an invisible "moveable" layer over the moveable parts of the playlist.
Would it work?
Wait, no, it wouldn't because when you click the playlist it will rise above the main window and the "lock" layer will be below the playlist.
Has anyone tried anything like this yet? I think the ability to build playlists into the skin itself is a must.