A hack is a novel solution using existing technology outside of its primary intended use case. Even though this is a 'social hack', a hack is nonetheless exactly what this is.
Perhaps 'trick' would be a better term in this airport case. I guess the difference between a trick and a hack is that you can't trick a machine (it does what it's told and doesn't care what your intentions are) but you can certainly trick people (they can be fooled in to misreading your intentions and responding dynamically). Also a trick is usually just about improving device utility rather than re-purposing it.