There is really no point in doing it.
Marijuana plants will grow straight towards the light and tomato's will grow anywhere as long as they get light. If you have ever grown tomatos then you will know that you have to stake the plants when it starts growing fruit or hang it like these people do. If not then they will fall over and the tomatos will rot on the ground.
If a pot plant was upside down and the light was from above it then it would make a U shape and grow upwards. I guess you could put the light on the floor but that would be kind of a bitch with watering and it would freak the roots out.
Just grow normally man!