The problem with CoinOPs/MAMEoX is that it maps the light gun as an analog joystick. It does the same thing with a trackball.
Okay, now think about a game like Centipede. Your little dude is at the bottom and you push the trackball left and CoinOps tells the game "push the stick left." You don't need an exact position, you just need a direction to go. The play, in my opinion, is fine.
That does not work in a light gun situation where you're pointing away from the screen or moving rapidly. There's a youtube video with an Xbox playing CoinOps with a Top Gun and notice that the guy barely moves and always returns to center. I'm skeptical that regular light CRT lightguns work at all with CoinOps. Extremely skeptical.
As for console emulators on the Xbox, I can't really comment. If you can point towards one specifically I'm willing to test it out.