Okay, I don't even know if this will work. It probably won't but if anyone can test this, it would be cool.
So, you know how zappers don't work on flatscreen TV sets, due to scaling lag? Well, why not have a "converter box" that connects to the Famicom's video output (which could be RGB, AV, or maybe even RF) as well as the Famicom expansion port, and then it detects when the screen goes black, WAITS for the zapper's photodiode to detect a black screen, then when the white square is shown it waits for the zapper's photodiode to detect it. The wait buffer would be about 1/4th of a second.
This should easily be able to be done with an arduino, an AV splitter, and some good programming, but then again I don't know much about these things.
You could even make a revision NESRGB that uses the data directly from the PPU to do that sort of thing.
Yeah...that was strange
I had a similar, but simpler idea where you just create a delay circuit of some sort to be put inside the zapper causing zappers sensing function to offset it's detection by about 40 milliseconds or so, which is usually about the length of lag caused by upscalers.
But, then would the NES be waiting for a response from the zapper by that time? I thought it only detected what the zapper was pointed at when it aired the white-square frame.
Then again, my idea wouldn't work if that was the case either.
You could also just have sort of a Wiimote-like device with the sensor bar, and then you could detect the zapper's position BEFORE the signal reaches the TV.