Looking at this from a historic perspective, this has been done many times in the past. It was not unusual for theatres in the late 30s, 40s and 50s to have a small confectionary store next to the lobby with the entrance from the street and a counter through the ajoining wall into the theatre lobby, thus fullfilling the theatres concession needs by way of that store. Sometimes the store was operated by the theatre owner, while other times the space was rented from the theatre owner to someone else who then in addition to paying rent for the store gave a percentage of the sales back to the theatre.
Although I am not aware of any new theatres still doing this, I do recall a theatre in West Virginia called the Robey that was still running a few years ago that had a window from the concession stand to the sidewalk.
I do get people who just want to come in and purchase pop corn or other refreshments from our stand which is beyond the ticket taker. We allow them to do that. As we are a single screen, it is pretty easy for the ticket taker to keep an eye on them to make sure they leave and not go into the auditorium.