I immediately knew that I wanted to use my favorite USB microcontroller, the PIC16F1455. It comes in packages with as few as 14 pins, or as small as a QFN-16, and requires no external components beyond a pair of simple bypass capacitors, making it perfect for small, simple USB devices. It's also supported by the free USB M-Stack, which means I'm not tied down by the frustrating license stipulations of the Microchip USB stack.
The real design revelation came when I tore apart a cheap $2 DealExtreme Bluetooth dongle to find that all of the electronics, including the actual USB pads, were all on a single PCB that could be easily removed from the shell.
The tricky part was that the PCB was 0.6mm thick, and finding a manufacturer willing to produce boards at that thickness for less than $100 took some doing. Once I realized SeeedStudio would handle such a board, it was a simple matter of measuring the original board and throwing together a replacement in EAGLE.
The firmware isn't quite done yet, but I do have the device enumerating as a keyboard and mouse and can send arbitrary mouse movements and button presses, as well as keyboard key presses, so all that really remains is setting up a queue-based event processor and then feeding it the original USBDriveby script. All in all, I'm pretty happy with how it turned out, and now I'm trying to come up with other ideas for how to use this thing, since I'm probably not going to get much use out of it as a MacOS exploit. The board has a single push button and LED (plus an additional power LED), so I can probably find another purpose for it eventually.