Think it would of been more believable if they made it hook up to the bottom of the iphone than using the audio jack to supposedly send out the video feed.
Without seeing a iPhone schematic and a BOM I really can't say if it matters or not. You might indeed be able to use the iPhone audio jack to output video signals. Or forget the audio jack altogether and simply feed the transmitted signal from your iPhone into a small RF amplifier.
What I do know is this: although the system shown in the video has apparently been debunked, is within the realm of possibility. There are no serious technical barriers to doing what is shown in the video.
You can find off the shelf components to build such a device for the phone as well as the required repeater. Or cobble together a dedicated unit from RF modules and Radio Shack components. When I was a UARC Member, I would sit in a room every week with Hams that are more than capable of building something to raise hell. And it is also quite easy to build amplifiers to boost the signals outputted by off the shelf RF modules.
I am not saying it is easy for everyone, just entirely possible.
Not sure how you get your repeater's signal into the video stream, however. It might be easier if these displays relied upon transmitted video and audio rather than a cable connection. Then it is a "simple" matter to inject a signal. Illegal as hell, but possible.
Not too difficult to use high power amateur equipment. These devices are readily available and outputting in the multi-kilowatt range. I know a guy that runs 1500 Watts of RF in his van. We can work all the way up to and above 300 GHz. Certainly nothing that can fit in your hand or ride the currents on a bright red balloon as depicted in the first video, however. Perhaps the VDs are so unprotected, they make small wattage workable.
The other possibility is this: the systems used by display manufacturers are too well protected to allow hacking as displayed in the video.
Bob