Mmmm, good one, never tried it myself. Thought maybe with an oc program like afterburner, you can manually set the fan speed. I just tried with mine but you can't get below a certain speed, meh.
Probably have to disconnect the powercable from the board ( if not solderd that is ) so you'll have to open the gpu case for it.
Sounds more and more as a stupid idea, never mind . . .