Hey everyone,
I just sat down at the DGX 650 and saw that Yamaha has added a third fingering mode called AI Fingered. "Less than three notes can be played to indicate the chords (based on the previously played chord, etc."
So I tried it and if I play a C with a G below it plays as a C/G and shows this in the screen. D with A and I get D/A, etc. I am not a piano player and not sure if this will do what you are all asking for but the older versions did not have this AI mode.