I dont think its possible on a capacitive screen to wake it up using a touch of the screen, unlike how resistive screens (like on an N97 or 5800) can do it. Basically its a difference of technologies.
1. Capacitive screens respond to your touches due to a distortion on the electromagnetic field of the screen, because the skin is also electrically conductive. This is also why normal styluses or fingernails dont work on capacitive screens (technically a sausage could work as a stylus though

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2. When a capacitive screen is off, usually the phone turns off the current to the conductive surface as well to save battery, thus there is no electrical current on the screen when it is asleep. There will be no electrical field to distort by your finger, thus the screen cant be woken up by pressing it.
3. Resisitive screens are able to do this because of the difference in technology. There is no electrical field in resisitive screens, only two layers of electrical conductors. When you press down on a part of the screen, it caused these two layers to meet, and the point where those two layers meet is registered as your touch. Hence it basically acts just like a normal keypad. The lockscreen switch is simply the on and off switch of these two conductor layers.
Basically, what you are asking from your phone is to basically not turn the screen off at all, which I would advice against.