It would seem that if you're modifying the cabinet to use a different type of monitor, you could also modify the gun. Is there some kind of light gun-like technology that does work on LCDs?
The issue is that you'd also have to modify the software. A traditional lightgun can't tell the game where it recorded a hit, only when. It's up to the software to translate the timing information it got from the lightgun into a position on the screen. It accomplishes this by being synchronized with the electron beam of the CRT and thus having an accurate model for the position of the beam over time.
VLSI CRTC's like the 6845 had a light pen input and a register that captured the character position at the time it fired. You could read that and translate it. It was a small matter of integration to make the input work. This chip was meant for text display but the PC and other systems used it for pixel-based displays too.
