It's nowhere near as good as it was in its hayday in 2006, but I wonder if, instead of specifically finding playlists with the song, you could instead grab 5 songs either side of it in scrobble histories. Sure, you'd have to filter out album tracklists, artist shuffles and then dedupe. But because of what last.fm is (99% a tool for listing everything you've ever listened to) it should have far more data to pick from, including all of your inspired ad-hoc mixes that would otherwise be lost.