Gaffa tape the iPhone to the passenger seat sunvisor. That way at the barrier you can have the sunvisor folded up and the phone won't be seen. Once through, pull the visor down and you can start videoing. It might be better to have the iPhone 'upside down', then when it comes to editing the video turn the image 180 degrees
If you want a facing camera to see the faces of the passengers, use a Brodit clip on the centre vents and just clip the phone into it. The marshals won't know the phone is being used as a camera.