I've got the same problem on my G46 and haven't seen anybody with a solution, or even acknowledge it as a problem... I'm glad I'm not alone ha! I do like to do some mobile gaming and it's exactly as you say, when I aim down the sights, I can't fire! External mouse obviously solves it but that isn't the solution I want all the time. I also have a Elantech touchpad. I am using the Elantech drivers from Asus.
I didn't realize it worked with the generic drivers, I'll have to try that. I was afraid it was a hardware limitation for some weird reason. I do hate the thought of losing the extra touch gestures.
Asus G46VW - i7-3630QM, Crucial m4 256GB mSATA SDD, 750GB HDD, 16GB Vengeance 1866, Nvidia 660mGTX
Asus G73JH-RBBX05 - Samsung 256GB 840 PRO SSD, WD 750GB Black 7.2K, 6GB, Bluray