That's about the same ideas I had too. Maybe I will eventually come back to this.
There was also a discussion about this in the A8 forum. (I'm just linking a post from me here:
Like said there, the Wii-mote is an insufficient replacement. Just change your pointing position or tilt and you can forget about absolute tracking the right spots. That should be the reason, why in the modding vid the guy sits relative statically on a chair pointing without big movements to the screen. Every movement out of his axis of calibration would need a recalibration...
I believe that is why the wiimote has the accelerometer and gyroscope in it, in addition to the IR emitter. There is a lot going on in a wiimote that is being transmitted by bluetooth to the Wii that is not being used in the video and could probably not be used without massive hardware and software support. I imagine the dolphin bar that uses the wiimote in PC games also has software that supports the gyroscope/accelerometer orientation or it has the same limitation. Most of the simple PC light guns like AimTrak act as a mouse or joystick and the manuals say people of much different height cannot easily use the same gun, and you cannot go from shooting at eye level to shooting from the hip like you can with a wiimote on a Wii. There are USB to Atari adapters that could fool an Atari into thinking a correctly designed gun is a mouse or joystick, but it cannot compute the relationship between a change in the angle of a gun and a change in the position of the gun unless it also has something like a gyroscope to sense gun angle and and programming to correct the "cursor" position accordingly.
However, a USB mouse-gun could work primitively with a USB-to-Atari mouse adapter, a sensor bar and an Atari console or A8, with the understanding that it's your hand position, not the pointing direction, that determines position. Some of them don't use any drivers. Then, your games would be like ST mouse hacks except the ST mouse is an optical mouse. The hardware for this exists, but I'm not sure the game programing could communicate calibration to the mouse-gun.
The alternative, slightly more old-school, possibly more fulfilling method would be some simple camera technology in the gun barrel like a CCD that detects a specific frequency from the screen at a certain threshold intensity. For instance, there was a UK PAL playstation2 eyetoy game called Hero (maybe?) where the person playing wields a real, physical, specially-colored neon-green sword that the eyetoy used for positioning a similar sword on the screen. Except in this case, the eyetoy would be a much simpler camera that is moved by the player and the "green-sword" would be the intermittent square(s) on the screen. Frequency detection by the CCD at the same time the trigger is pulled results in a hit. You might even be able to make a simple digital to analog converter by causing the CCD to set off an LED for the actual gun sensor. Like in one of the discussions I read, it would probably work better in a dark room. (LCD pattern)-to-(CCD)-to-(LED)-to-(original gun photo detector)-to-atari.
Even more old-school (gun and console stay the same), while ironically, more Star Trek-y, would be to invent some clear screen you put over your LCD screen that converts LCD-like black and white emissions into more CRT-like black and white emissions via florescence. But you might as well set a CRT next to your LCD before getting to this point.