I have a neat workaround. Sort your songs in smart playlists in iTunes but not just by key, instead after what songs work with a certain key. For example, I have folder 5A in which all songs appear that are in 4A, 5A, 6A and 5B. When I choose the next song I check what key I'm in, enter that folder and can immediately see what tracks work with the current one. I also have a "boost" folder for each key. The really neat thing is that every time I add a track to iTunes it automatically populates the playlists, all I have to do is mark the key in the comments field. Then I delete all playlists from Rekordbox and re-import via bridge from iTunes. Sounds complicated but works like a charm once you've set it up!