Now you made me think horrible thoughts: What if the signals to the LCD happen to be AC coupled into the cartridge pins enough that you can filter them out and

Now you made me think horrible thoughts: What if the signals to the LCD happen to be AC coupled into the cartridge pins enough that you can filter them out and reconstruct the image from it, at least with some level of fidelity. Probably not though. For starters, the interface to the LCD is most likely parallel, and you have to be really lucky for them to be discernible over unintentional coupling. For example, it could still work if the individual digital signals have slightly different phase and amplitude, in which case what you get from the cartridge pins is "just" quadrature amplitude modulated multiplexing of all pins. And you'd need some high quality test equipment to sample that...