It would sound to me the Polaroid back was experimental, rather than workable. As Nick Merrett pointed out, the 6x7 / 67 bodies require film to be loaded in the chamber and tension placed on the take-up spool in order to effect operation of the shutter. That's how the internal mechanics have been designed. The 6x7 / 67 bodies were never optioned with any type of film backs because of the complexity of the winding and tension design.
In normal circumstances, a 6x7 / 67 body that does not fire can have any one of a flat battery, jammed mirror, slipped winding pawl (that prevents the shutter being re-cocked) or some other malaise. The recessed button on the right side of the camera is used to reset the cocking mechanism (at the expense of a wasted frame). For the Polarioid film back to work, it would need to interface with the take-up spool of the camera. If it doesn't, the idea was simply a lightbulb moment with no workable way of firing the camera. I could be wrong, yes, but quite a few people have taken an axe and claw to these otherwise steadfastly remarkable cameras by removing backs and tweaking the internal business only to end up with a camera that will not do a single thing.