This evening, having found myself in possession of a MIDI numeric keypad (which makes it so much easier to issue commands to the MIDI hardware), I put my theories to test on an as-yet-unmodified keyboard.  I replicated the ribbon-cable arrangement from the choir manual, but left the busbar unmodified.  Cycling through the four configurations possible on the keyboard scanner card, I was able to confirm that configuration 3 is the correct one, that the ribbon-cable arrangement is correct, and that I do in fact need to adjust the bus-bar.

So the plan for tomorrow is to secure the ribbon cable on the choir manual, and complete the modifications to the great.  I will try and do the swell on Friday, but the weekend is looking to be very busy.  Next week I will think about where to affix the MIDI modules, and also start work on the piston wiring.