Someone has been kind enough to share this important info, and has confirmed that the NA version of Ghost Squad Wii does indeed have the option to use the Z trigger on the Zapper. He also mentions that the calibration while not perfect is pretty good.
I picked up an American copy of Ghost Squad today, and you can -- I repeat CAN -- use the Z button for firing when using the zapper. It's one of four configs. Front trigger/A is action and Analog/D-Pad is weapon select under this setup.
And I must say that if you calibrate the zapper before playing and turn off your aiming reticle, you get a damn good representation of the light-gun experience. You just can't go moving all over the place when you play, or the calibration ends up being off. - hclflow
I didn't have a lot of hope that it would, but I'm very glad to hear that it does use the Z trigger. It makes no sense that the Japanese version would not allow this, but whatever. Even better news is that the calibration works well. Can't wait to try it out when I get my copy in the mail.
This news is even better for the upcoming House of the Dead compilation the Wii is getting next year!