There's actually a gun grip that works rather well for this sort of thing. I've seen it under several brand names so I'm guessing it's just a generic thing made by one company and distributed by several others. You can buy them at Dollar General stores (at least, that's where I found mine) for five dollars. I got a pair to use for Ghostbusters: The Video Game and they also work surprisingly well with Metroid Prime 3: Corruption. Here's an image: http://image.dhgate.com/upload/20084/57/ff80808116c6e66a0116c96e3c590d9d/productimg1207637727124.jpg
The design allows you to grip the front as you would a sawed-off shotgun, which means most people can comfortably still hit the D-Pad and A Button on the Wii Remote with slight stretching required to hit the 1 and 2 Buttons. If you can find one at a Dollar General or other discount store, give it a try.
well, feel free to use the Zapper, it might make it more difficult though... I would suggest if you want to use a Zapper with an RE game go for Umbrella or Darkside chronicles... one of the rail shooters.