Since Arduino can be programmed to function as a keyboard[1], wouldn't that be an easier option? Then you can put the chording logic on the Arduino board and have it function like a plain USB keyboard otherwise as far as the computer is concerned[2] (that's kind of how the Makey Makey[3] works - it's also Arduino based).
[1] http://arduino.cc/en/Reference/KeyboardWrite
[2] http://store.arduino.cc/index.php?main_page=product_info&cPa...