Food is allowed on a case-by-case basis. I wouldn't expect bentos to be problematic in this regard given the nature of the food.
The Marriott won't allow any cooking in the panel rooms because they're potential (if unlikely) fire hazards, however, so any items that would need to be cooked would have to be cooked in advance. Additionally, con policy against bladed weapons would still stand so you couldn't bring any knives to the panel. That would pretty much limit it to assembly of the finished product.
And finally, our contract with the hotel prohibits us charging for food. If there's any food that you'd like to share with the panel attendees, it must be provided gratis. (The con also could not reimburse you for this expense for that same reason.)
