Apple's touch pad uses custom IC to read electrostatic capacitance from the pad. I could not find datasheet for the IC.
Linux uses synaptics driver on Mac PC.
I forgot well about the IC but it might be Broadcom BCM5974 which reads electrostatic capacitance. If so, you can find some driver code in the Linux kernel.
Some people successfully converted old 4pin touch pad into USB touchpad. But recent one has 22 pins. It seems the USB controller is placed on the mainboard for recent version.