Weed is Gravitropic. So it wont work...
http://en.wikipedia.org/wiki/Gravitropism
The plant will bend up towards the roof no matter where the light is.
Fan leafs are Phototropic, they follow light, but the plant is gravitropic. Theres these starchy cells that settle in the roots so if you tried to grow it upside down these starchy cells would settle in the tops of the plants and would fuck it up.
Also roots grow in search of water, unlike tomatoes, to grow weed upside down you would need the pot upside down or else some INSANE bending and LST.