Hello Tom,
Registration are embedded into the midi file. While it is possible to get a printout of a registration file (once loaded in Hauptwerk go to Registrations \Export/View...) a MIDI file will send the needed MIDI messages as CC (control change) during the playback.
Here is the MIDI messages transmitted when pressing the Great piston #3 on St-Anne (with its default registration file loaded)
So not really easy to understand