I am not exactly clear on your application or rather the specific need for what you are trying to do.

You can not make page flips conditional on a status test... at least not yet.

I would suggest using a transformer to do a voltage sense out of the outlet on the back of the box rather than a video sense. You can switch the behavior of the outlet in the cable box menu (at least with most boxes depending on the firmware).

Personally, I just leave the boxes on. I find far fewer issues with this method because automated firmware changes can reset setting you may have made (like the switched outlet).

If the box is off is it not correct to assume that the client is not engaged in a viewing activity and thay must choose a new activity to view cable? Why can't you make sure the box comes on in the cable activity macro?