In my Skype session with Hellene last week, she mentioned the Solfeggio and Chords section of the Gentle Piano library. She suggested I set the staff presentation to 6 and hide the left hand, and after playing the melody for the student, asking the student to play the left hand (hidden) for harmonization training.
Since harmonization and melodic dictation are already part of my teaching curriculum, this will fit right in. I have a dyad lesson (two students together) this afternoon, and we will be using the program to practice harmonizing in the keys of C and F (which they are currently studying).
Here is a short video of my daughter harmonizing Hot Cross Buns, just as an example. She already knows the piece, but here she has the staff set to 6 (usually she is on 1):