I think we should go with what has been done in many previous DebConfs: food tickets.
I personally hate the paper wristbands, I usually can't stand them for one day, I would go crazy having to wear one for two weeks.
Tickets are not particularly awesome, but most people handle them alright.
The problem is that in that case we have to pay for not eaten meals. What we can do (also done before) is ask people who are not staying for lunch or dinner to give up their tickets, so that they can get sold to someone else (main target would be passers-by that haven't registered).
We will need a way of identifying who has food (either paid or sponsored) anyway. The wristbands might be easier to check, but they are harder to transfer, and I think it makes sense to make the food token transferable, so that we can sell them.
The other idea was to reduce the number of meals we order. We know that as the week goes on we'll have less diners, but it's hard to take advantage of this without risking leaving people without food. Let's say it rains one night, when we had estimated 90% people, but because of the rain, 95% stay to eat. What do we do about the 5% unaccounted?
I think it's better to go with what we know for sure, and try to sell what we can to outsiders.