I don't think that a food mechanic would work in such a game as this.. If your not online for a couple of day's, and your people are dying because there is no food, when you come online you have no army's anymore, is it than fun to play the game after such a thing happens? I played a lot of Settlers 7 and the online matches are very very long.. But even if you don't pay atention for a coulpe of minuts, everything is compleetly messed up..
So I think its better to have an AoE style of food (gather the food, spend it on units and tech) then such a mechanic. And siege would work with this system also.. Because if you have no food, you can't train anymore units, so your lost..