Modifier Keys Across USB
I ended up creating my own "keyboard combiner". It lets me use the Kinesis Savant Elite Triple Foot Pedals for shift, alt, and command modifiers on a Mac.

https://github.com/davidbkemp/KeyboardCombinerArduinoSketch/wiki

It is quite easy to assemble, but I cannot guarantee that it will work for you. It certainly does not work with some pedals and some keyboards. For the Kinesis pedals, you will initially need to configure them to act as shift, alt, and command keys, and that means using a PC to do the initial configuration.