When I worked at Burger King, when I first started, we would serve people on foot in the drive thru. We hated doing it, though, because there was no way to trigger the speakerbox on foot (it had the same sort of magnetic sensor older stoplights had), so it would mean that the orders wouldn't be made in the order they were received. Most of these happened when we were open inside, so I never understood what people were trying to accomplish (they thought they were being funny, probably).
Later, when we extended our hours to 2am, we were told by corporate this wasn't allowed. None of us really argued much.