I've been thinking about this for a while, and thanks to some pointers from the Synth DIY mailing list, I've managed to get a nice workable mod for realtime filter cutoff and resonance on my Poly 61.
The digital signal for the filter cutoff can be found coming out IC 42, where a simple DAC is created with 6 signals and some resistors. These lead to IC 36, which buffers the signal and sends it to pin 12 of connector 10A (cpu to voice board).
Similarly, filter resonance comes from IC 36, a buffer that is connected to IC 28. Pin 13 is the location on the connector.
Take the signals from the connector cable by carefully cutting wires 12 and 13 and stripping them. Solder wires and insulate propers (this can be tricky, and messy).
For the cutoff pot, you can create a voltage divider between +5 volts and the signal coming from the CPU board (max cutoff is -5volts). There are many places to get 5 volts, I used Pin 26 on IC28.
For the resonance, use a ground point for one side of the pot, and the cpu board side for the other, the signal is 0-10 volts.
Probably not the cleanest mod in the world, but simple to do, and it really adds a new level of expression to the cheap and easy to find Korg Poly 61. One note, when you program a patch now, the cutoff level will determine the max cutoff amount the potentiometer will affect, and resonance amount will determine the minimum resonance setting for that potentiometer. In other words, for full range, set the cutoff to 63, and the resonance to 0.
Enjoy non stepped filter sweeps!