After careful consideration, I have decided to adopt the Arduino shield form factor for the breakout boards, and to add one dedicated Arduino Uno to every polyphonic channel. This will make it easier to find off-the-shelf sub-modules, and it will bring a ton of features for free. As a result, the number of analog ports per channel goes down from 8 to 6. In exchange, it dramatically simplifies the architecture for dynamic patching, since all Arduino pins will be statically exposed onto the breakout boards. Consequently, the minimum number of analog crosspoint switches goes down from 32 to 8, and the projected retail price goes down from $1,995 to $1,495.
An updated presentation is available at ModularGrid.