I had ordered the emergency stop when I bought the first round of parts for Maxwell, but I hadn't actually designed a location to attach it to on the robot. This weekend I rebuild a number of components on Maxwell, installing a new base, upgrading the torso wiring harnesses, and installing the E-stop (seen behind the arm, protected by the torso column):
The E-stop is from Digikey and is the type it latches when pressed, such that you need to twist it to release and restart the current flowing.
Maxwell's ArbotiX is powered off the USB connection, allowing all 12V power to run through the E-Stop. Pressing the E-Stop then stops all the motors and servos, while the ArbotiX continues to run and interact with the PC. On releasing the E-stop, everything comes back to life nicely.