You can use a plastic bag with a hole and rubber band for the lens. Personnally, if I feel the need for a for a plastic for shooting, I just keep going. Stopping and taking the camera out (and back in and restarting) in the rain is never fun, no matter what your setup is.
Use a rain cover for the bag! When I say my camera never got wet, it's with the rain cover. What I meant was that you don't need an immersion-proof bag... unless you plan on going swimming with your camera. I'm not crazy about watertight panniers but I wish Arkel made their handlebar bags rainproof. Just a waterproof fabric with a big flap over the zipper would be enough.