Hello Jon,
I don't have experience with the Peterborough sample set, so I'll leave others to comment on whether it has any dedicated functionality (e.g. MIDI-mappable sliders) for controlling the amplitudes of divisions (aside from its swell boxes).
Within Hauptwerk in general (independently of the sample set's functionality), you can adjust the amplitudes of ranks/pipes via the voicing screen, but there is no way to do that, or to do anything equivalent, via MIDI. The only MIDI-mappable amplitude control is the overall 'Volume' slider.
[I'll move this topic to the "Technical support" section of the forum once you've had a chance to read it.]