Could have made a really great song.
I know there was a rumour that when Em was making Relapse he asked her to collab and she turned it down. This rumour was in The Sun, known for it's complete bullshit, so hardly a reliable source, but I'd have loved that to have happened.
Obviously, both had well publicised drug problems and, like Em, she is quite a dark, clever songwriter/lyricist, so I think they could have come up with something creative surrounding that theme. Probably wouldn't have fit on Relapse, though.
Shame, really.