This seems only to be implemented in the old analog yamaha cs80, cs 60. would be a first!!!!!
Glissando Portamento Effect
pugface: Do you mean that the glide should be in semitone steps?
Since you've mentioned it, I'd like to add that it would be nice to be able to select if the portamento knob controls the glide time (as it does right now) or the glide slope (the time to reach the new note increases, as the note distances increase, but the pace at which it is reached stays the same, and is controlled by the knob).
It's not exactly what you are after but if you play around with the "smooth modes" under the "common" tab in VC you have a controllable stepped style effect for when you use your pitch bend wheel or turn knobs. Try increasing the bender ranges for maximum effect.