Hi Tim!
Depending on your setup there could be more or maybe only one solution.
If you only have a DMX channel to control the screen you have to measure the time scrolling up and down the screen, as this differs in both directions. This way you would have to know the time which it takes from e.g. 4:3 to 16:9 and all other combinations. These timing values could be programmed onto the timeline as well, but will only work when you run your show from start to the end.
If your screen does not have any kind of DMX control you could as well go for a relay switch (from Phidgets) to power the screens up.
Attaching a sensor to the screens or using a maybe already installed one could offer you the possibility to readout the values and stop at a defined position as soon as the value is reached.
Do you have a chance to get encoders working on the axis? Do you have DMX control?
Markus