I am just curious, how did it work out?
I am also planning to buy an ErgoDox (the second batch will be available soon) and I am looking for a solution to use pedals as modifier keys on Mac OS X.
On Mac OS X the internal do not support modifiers from one input device to work with a second input device. Therefore the modifier should be sent through the same device (i.e. ErgoDox).
An other idea would be to build a "man-in-the-middle" device with a Teensy board. It would just forward the signals coming from the keyboard, but "add" the modifier keys coming from the pedals.