You could try to force the ImagePro to send a certain EDID. Instead of just selecting an input resolution - you can programm a fixed resolution that will be trasmitted to the Mac as EDID. - You can even achieve resolutions (like 1920x1080) that would not be available otherwise.
In factory setup the ImagePro will pass through the EDID info of a connected monitor (on the DVI thru port)...