Long time, yeah? But it was necessary.
Finally I can announce something wonderful!
I've found a solution to play zapper games on emulator. With actual light gun!
I will publish DIY instructions as I come up with general solution.
My method still depends on modified roms though. But this is huge for those who can't afford flash cartridge, and of course for development purposes too.
If you want to make your own zapper game, feel free to contact me for advice.