You won't be able to get food in exchange for your beverage allottment.
I'm actually shocked that anyplace allowed an appetizer instead of a dessert. That's been a big no ever since the dining plan was split into separate plans and the appetizer removed from the regular plan.
Sometimes you don't get offered a substitute like fruit, but if you ask they will come up with something. No guarantees though, and it's going to mean that not every restaurant will allow you to make such a substitution.