I think its a very cool project. But I am a little confused, that mouse implimentation uses the ADNS-2610 which is a currently available part costing around $1.50 in one off quantities.
It uses two wire synchronous serial communications to access all pixels on a chip so I think one chip would give you 324 pixels and you could use four chips (five pins?) if you needed more pixels. Perhaps the problem that may rule this very low cost solution out for your application is the optics necessary to get the mouse sensor to read over the full area of the display. But it sure would be a low cost way of implementing your application if it could be done.
If you do want to use individual sensors then some form of multiplexing technology that allowed you to select sensors, perhaps reading 8 at a time as you 'scanned across' the sensors behind the display would be the way to go.