This is something I have always wanted to do. I don't know if I can pull this off (contraints are time, and energy). There would be two main components. A controller card, that would interface between a Midi Interface and the voice cards. This would be a huge projects.
Anyway, here is a sketch of a block diagram for the voice card. I consider this to be the hard part of the project. It will be a voltage controlled card, although, the voltages will be supplied by an on card DAC and a bunch of sample/hold circuits. Of course, there will be a tuning problem with the VCO's. Especially if they are exponential VCO's.
If you have any suggestions for features, speak up.
It will be a while before any of this gets started (if ever).
All for now.