If the nut is blocking the main air direction control, it is most likley caught near a internal air ducting flap, get a torch and have a look/poke around under the dash ,look up through the holes where the heat would come out to blow on your feet.
You may see some holes that allow restricted access to the lower flap/s, i normally use a "large" fat cable tie to poke (you can also bend it to get into corners) or just something that you know wont damage the heater core etc.
Blowing compressed air up down and all around in holes and the vents, at the same time moving the flaps/controls, is another suggestion worth trying.
If you do see the nut, you can use a bit of chewing gum on a coathanger (or somthing sticky) to pick it up, that way you dont knock it further inside by mistake
The GU dash/radio/vents surround panel is easy to get off, this will give better access to the top vent tubes as well