When speaking in the context of Photography... is it legitimate to refer to the film or sensor as occupying the Focal Plane?
I would say, yes. Even though they are different things.
After here, sensor will mean digital image sensor or film.
Referring to my answer here, the position of the sensor matters! You can focus on something by moving the sensor back and forth, while having the lens stationary. You are adjusting the focal points by moving the focal plane, in which case can be the sensor (or image plane).
As an example, most prime lenses for film cameras, turning the focus ring will move all lens elements away/towards the sensor (or image plane). So, moving the image plane (or sensor) away/towards the lens elements will have the same effect.
I've actually done this on my OM-D and an OM 50mm f/1.4 lens. I turn the focussing ring over to it's minimum focussing distance, cup one hand over the back of the lens an hold it down on the table while moving the OM-D back and forth with my other hand.
There I saw it. It had the same effect! (As focussing the conventional way) Although, not very practical.
EDIT: But, I guess in terms of photography, we adjust the focal plane with the lens. We don't adjust it with the image plane.