Quite a few things are missing from the point-of-view of a 'strict' H910 emulation. I look at that PF algorithm along the lines of what more it can do than the original hardware units. There's no way to access antifeedback over MIDI, and I can't speak to any future plans that Eventide might have in this regard.
An external frequency shifter of a few Hertz might do the trick. Electro-Harmonix (legacy Frequency Shifter or the new Ring Thing design) comes to mind. Maybe a slow & low depth TC Shaker or E-H Wiggler (any true vibrato) pedal would cover it. Those boxes won't cover stereo / both channels of the PitchFactor, though. I'm curious: What do you have in mind for the application of such a feature?