You would have to book an ADR/arrive when they are taking walkups (non-peak times, maybe this would work) and be seated to order one.
I don't know what their to-go abilities would be (i.e., whether they have any paper/styrofoam cups on hand), but we have actually booked an ADR just for milkshakes/ice cream dessert in mid-afternoon, and it didn't seem to bother the server in the least that we didn't order any food.