How do you assign Polyphonic Aftertouch to control a parameter?

I'm using a Roland Handsonic HPD-15 as a midi controller. Each of the 15 pads sends polyphonic aftertouch.

The poly aftertouch shows up in the midi message monitor and in the activity monitor windows. But in the Layout window when I bring in a knob/fader/meter and I try to assign Midi functionality to it, Poly Aftertouch is not listed in the list of Midi controllers.

I'm a bit worried that using the "Scripter" feature in MIDI FX might be my only option?