Meanwhile, since I have a tech background, I'm also interested in mods. I've done a bunch to an old Yamaha CSY-2AE organ (contains the 2nd generation of their SY-1 topology), so I'm reasonably familiar with the standard constructs. (S&H, Ring Mod with dynamic VCO, Moog Ladder Filter, Pink/White Analog noise, mixer, etc.)
I have yet to look into the Mono/Poly design in a big way, but the most obvious mod would be to do the polyphonic VCF/VCA that you mentioned was something that you were working on? I may have missed related posted updates but wanted to know how this worked out for you? I have a bunch of IR3109's filter chips from an old Roland unit, ideally configured as state variable design. Individual ADSR's would be nice...
The main issue that I can see might be the keyboard tracking, but I gather that can be picked up off the corresponding VCO?
I have a spare keyboard off a CSY synth/organ with mechanical 'aftertouch' so I expect to use that in place of the missing keyboard of the Mono/Poly to provide real time control of whatever c/v parameter.
Any new suggestions since your initial considerations would be good to know, as well as any practical tips or precautions you've come across.
Thanks!