Just an Idea, here but could it not be possible to do some form of flash rom cart that could be pluged into the nes, with a connector leading from cart to pc (possibly usb) (the nes lid would need to be held open like the action reply did, so that the wire isn't damaged) this way demos can be tested on a real nes without doing anything to the nes (which I think should be cheaper) and using some form of mapper emulation - at least the common mappers could be supported or worst case it could be a simple mapper 0 tester.
If so, maybe someone could do this - electronics are't my thing, so I could't if its possible.