So if it has a customer assigned this designates it as To Go? You do not assign customers otherwise?
EDIT: Nevermind I see you already said this. My restaurant operates like this as well. I use Ticket State and it automatically changes State when Customer Entity is assigned. I use Entity Updated rule to capture it. This can also swap price lists for entire order using Change Price List Action
Basically when a Customer Entity is added to a ticket it changes Ticket State to Takeaway or To-Go. This in turn changes the Price List to my Takeaway or To-Go list. Technically you would not need to use Ticket State… simply having the check for Entity Updated and looking for Customer Entity would be enough to load a Takeaway price list.
I like having the State so I can do other functions as well particularly my Auto Print functions.