so there was a great thread on this in the archived mods forum, but unfortunately all the pictures were gone. thankfully i have an SSK II with TP I can pull apart to show you how it's usually wired up, but the basic idea is that the strain gauge and the encoder are on one chip, and then that has a flex cable that wires it to the main keyboard and mouse controller, which presents a PS/2 mouse to the host device.
i believe this is how it's wired up on all the modern lenovos as well. the flex cable ends in a ZIF, but if someone were to reverse engineer it, i bet interfacing with it and providing a usb mouse could be engineered into a gp micro platform like the teensy or arduino boards..