I got in touch with one of the designers for this project (Amit Zoran), here is what he had to say about the electronics involved:
"The first version used linear analog processing:
I am summing the signals (coming from 5 piezo ceramic sensors) and then amplifying the important frequencies for acoustic guitar's sound (110Hz & 220Hz are the resonance freq. - coming from the Helmholtz resonance and the size of the sound board). I am attenuating the resonance frequency that the new sound board (my resonator) create (500Hz). You can see it as a zero order interpolation for the 5 signals (each of the signals covers different vibration modes) and then linear filter - an equalizer.
For the new guitar prototype, the one in the top picture of my web site, I intend to use Freescale Symphony™ DSP56371 digital signal processor with their Symphony SoundBite Development Kit and to develop in C (with Symphony™ Studio software tools). For that guitar, the locations' of the sensors were chosen to cover the important vibration modes. The idea is to try different digital processing approaches - linear filtering / finite element based approaches / sound synthesis based methods / well known digital effects for reverb.
The best way will be defined in the near future."