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?