Hmmm damn what a YUK size...

I know its been done though on the factory screen as I have read it previously.
Perhaps email digitalww and ask them for the exact measurements of the 5.6" overlay.
I haven't dissassembled one of those so I don't know if you could physically fit a 7" overlay in behind, but that would be OK if you could because when you actually calibrate the overlay, you would only be calibrating the boundaries of your 5.6".
http://www.digitalww.com/56TSK.htm