In two networks I worked with, we were using the song key as one of the parameters for Selector.
What we did was hire a musician who had this ability to detect the key. So, we would tell Selector "that songs starts in A-Major and ends in F-minor". And then that same musician had given us a compatibility list. (That key can work with those keys but not with those keys, etc.)
We were also coding the jingles the same way, and they were a great help for key transitions.
Of course, this approach would work mostly for Gold / AC stations, and obviously not for CHR where the rotations are so tight that you pretty much don't have the choice of "what to play next"