Neat Idea.
I'm guessing they have a microcontroller with a A/D attached to the depth/wave knob and a D/A generating a control signal to the VCA. Then you could take periodic samples from an A/D and store them in a lookup table. Then step through the lookup table during playback and write the value to the D/A. My guess is the samplerate they use is pretty low and they uses some interpolation to get a smooth output. During playback the A/D would be sampling a value for tremolo depth that would be used as a mulitplier for the output value of the interpolater.
You could probably implement the whole thing on a PIC and use a resistor ladder for the D/A.
It would be nice if they added midi sync too.