jemplode didn't seem to do that when I tried earlier...is there a particular setting required?

Using Jemplode 41, I simply right clicked on a track and selected "Toggle Color". All parent playlists that contained said track changed color.

The main criterion I am using to discard stuff was how many (or rather, how few) playlists it belonged to

In which case, as suggested earlier in the thread, all you need is to look at the REFS field for each track. You can add it as a column in the detail view.