Some of Wacom's tablets can be used with both the pen or with your finger (acting like a touchpad).

Anyone have any idea whether the touchpad part could be made open-source? Or even some closed sourced off-the-shelf solution that could be integrated with the above?

EDIT: There is a Canadian company that has recently released an open source trackpad called the Ploopy Trackpad [1].

[1] https://github.com/ploopyco/trackpad https://ploopy.co/trackpad/