Not having any idea how they pulled this one off, and not being a linux kernel-hacker myself, I'm totally just guessing at this one. But I suspect the work is slightly more than trivial. I remember the USB Gadget Driver/Multi-function Composite Driver will let you do HID-anything-you-want-to-define including keyboard/mouse combos, but I'm not a C/C++ programmer, so I'm of little use there.
That said, I can't wait for mine to arrive so I can start testing. Payloads are something I can look at, as it's running Debian, and I can do plenty with that...