@MarkT: PWMing the rectifier output would surely involve modifying the innards. I'd rather explore other avenues first.
@Runaway Pancake: Interestingly enough, it seems to work fine if I plug it into this router/fan control: http://www.amazon.com/gp/product/B000HQAVNI. It doesn't get up much speed at lower settings, and with the knob turned right up it doesn't quite reach the same speed as on high, but it's acceptable - I'll put it down to energy loss in the controller. The only question is whether I can achieve some similar sort of control, but automated using an arduino...
Are these things usually just triacs?
mm
Edit: D'ya reckon I could modify or create something similar to this board? Velleman Multifunction Dimmer Kit K8028