The Arduino Mega 2560 Pro Mini has arrived. The good news is that it is compatible with the External Lamp Field. Below is the Mega sitting on its spot on the simulator. On the actual PCB, it will be soldered underneath the board. The USB connector will stick out through a discreet hole on the back of the simulator.
This device from miniduino matches the Fritzing footprint and uses an ATmega16u2 for the USB connectivity. It can be found on ebay by searching for "mega 2560 pro mini atmega16u2"
While this one is a little cheaper and works fine for other uses, the external lamp field does not have drivers for the USB chip on board this one: