shooting agains a video wall
I can't remember his name but one fairly well know professional portrait/fashion photographer shoots almost all of his stuff using a digital projected background (still, not moving as in a video) so it is possible to do what you are being asked to do.
Have you asked you client if they can select frames that they want used as the backgrounds and project them as stills. That would make it possible to do more than one shot of the model with each background, giving you a chance to change the model's pose and expression, or get a backup shot in case the model blinked.
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