Whilst a slightly different approach, Quod Libet has supported features like this for a while now (I am one of the devs). Using its slightly mind-bending search syntax:
...would return everything in an Iron Maiden playlist, but:
~playlists=|(Long Songs, Short Songs, Party Favourites)
...would return everything in any of those, and:
&(artist=Iron Maiden,~playlists="Long Songs")
...would return everything by Iron Maiden featuring in the Long Songs playlist, but you could do this automatically by:
&(artist=Iron Maiden, #(length > 300))
...which returns all songs by Iron Maiden over 5 minutes (300 seconds) long. Magic!