I haven't followed the link but I assume you are talking about the cute littel yellow and black number that costs about £30.
If so, is is actually an OWI EDGE 535 arm, and there is quite a fair bit of info on it on the net. The most interesting of which is the Instructables article that explains how to get reasonably accurate joint position information by hacking the servos (which are, sadly, normal electric motors, not proper servos). Now, given the whole thiong is driven my motors, and the board itself has a couple of bog standard motor driver ICs on it (I know, i've seen them), I think it would be fairly easy to, say, wire the GPIO pins on the Pi directly to the inputs of the driver ICs on the arm's PCB, bypassing the USB port.
Suddenly, you don't need a USB driver.... and I actually bought an Arduino for this purpose, but have not had the time yet to do it.