I modified the trackpad synthesizer I wrote to be a MIDI controller. It presents itself as a virtual midi source to osx. It has polyphonic aftertouch, in theory, but my synth doesn't seem to do anything with aftertouch messages. I've updated the code on github to be midi by default and only give skini output with a -s flag. You might need to install a virtual synthesizer like SimpleSynth and set it to use "touchpad" as an input to hear anything.

I looked into packaging it up as a mac app "bundle", but it seemed like a lot of work. If it's not working to compile, though, you could try the binary.