Ha! You read my mind! I had a third party wii nunchuk with a crappy joystick, so I didn't mind using it for this. I ran the nunchuk cable out of the bottom of the gun, and I'm using a wireless wii accessory to usb adapter by mayflash. Then I used joy2key to map the nunchuk Z button to the L-Click.
As for the infrared light, I got an IR LED from radio shack for about 2 dollars, and one of those long necked LED book lights from the dollar store. I swapped the book light LED with the IR LED and ran it through the barrel of the gun. I filed a rectangular opening into the top of the gun casing, and fit the on/off switch for the book light into it.
I did all this with one of those after market (retroware, I think) zappers. I didn't want to experiment with the real deal, until I was sure it worked.
I modified the plexmouse glovepie script - inverted the Y axis and multiplied mouse movement by 5 in order to increase the targetable area on the tv screen. It works pretty well. Although I'm thinking some reflective tape and/or a second IR LED in the barrel might help with tracking gun movement (especially when aimed at an angle).
The only big change I'd want to make in my second draft would be leaving the Zapper trigger in place. I was unable to do that with the third party zapper I used in my first attempt (it had an extremely shoddy trigger mechanism, compared to the original). Right now, I just have the two nunchuk trigger buttons in place of the plastic trigger.