I managed to send touchpad inputs using a button and two axes (a mouse). The button controls the finger presence, and the axes control the finger coordinates. This allows me to control the drone modes in Killzone! The source code needs some refactoring before landing into the git repository.
A few products (venom x, xim4, titan) have announced PS4 support over USB. They are mixing the USB transfers from the Hori pad and from the DS4. They all lack touchpad support, and there is no evidence touchpad inputs can work over USB.