Alan,
Thank you for your Post, as I had the same question regarding Auto Harmonize on my PX-780 and after playing with it for a while, I noticed that I had to turn on Auto Accompaniment in order for it to work and the Manual forgot to mention that, and Yes I also get that cheesy sound from the left hand chords which I wished it wasn't there and I was hoping there was a solution for that, but after reading this post sounds like I have to live with it. I'm just a beginner and trying to learn both the keyboard and playing it.
Thanks again for this post.