OK, Craig, here's what I came up with. Simplest configuration: Rapture or Dimension Pro microhost; Initialize Program; sine.wav Multisample; one Matrix line using Polyphonic Aftertouch as a Source; appropriate Depth for a select Destination. Polyphonic aftertouch transmission confirmed in Bome's SendSX before testing.
In either synth (untested in Rapture Pro), a Pitch destination to a single Element was easy to discriminate. Two notes with varying PAT pressure seemed to 'fight' each other for control. Lots of warbling before the firmest pressure won out (set to an octave up).
With a Pitch LFO Freq destination, the first note triggered seemed to take precedence. That is, the first note would have to be released before the 2nd note (with additional pressure) took over. So, it didn't really accumulate like Channel Aftertouch would. It was more of a tradeoff between notes and current polyphonic pressure.
I should mention here that the AKAI MAX49 sends a 'true' Note Off message with a Release Velocity of 0.
Rapture (but not Dimension Pro) got a little stranger. There were occasions where changing Destinations would 'stick' on the original Destination, until - oddly - another wavetable oscillator was selected.
Very preliminary testing here, and hardly conclusive. But thanks for reviving the questions that I've had on the "how's & why's" this should work.