Interesting idea Whit. From the PDP-1 Handbook:
It looks like the light pen had a photodiode that would trigger if a "dot" was just plotted at the point the pen was touching on the screen. This would trigger the Light Pen status bit to be set to one. So it would have been the programmers responsibility to draw dots across the screen in a "CRT scanline fashion" waiting for the Light Pen bit to be set then saving off the X (AC) and Y (I0) coordinates when detected.
The right way to do this would be to make a functional light pen and wire it into the PiDP-1 hardware. You could cheat and maybe use the touch screen interface and coordinate that with the virtual display to achieve the same effect. In either case the PiDP-1 team would have to change the software.
Oscar, Angelo have you considered doing something like this?
Mike