If you drive the pipes at the 'division' (=wind-chest) level in that way then you wouldn't even need any ExternalRank or StopRank entries at all. You would just have a Stop entry for each real rank of pipes and use MIDI output from it directly to engage or disengage the real rank of pipes as a whole (as with a slider wind-chest).
OK ... this is the way it's going to be since I can use that method for almost all of the ranks. It's the easiest method to wire, I think. Use a high-current MIDI driver board to supply +VDC to chosen notes on any keyboard. For example, since there are
seven 8' ranks in the great, I would wire together all notes
of the same pitch together, and those seven wires would be connected to one output on the MIDI driver board.
As each stop is chosen in this division, just ground the common of the chosen rank, and, voilà, the required note sounds while the others won't because the common side of the rank is not grounded (it's left floating, close to the +VDC). I assume Hauptwerk will look after the coupling.
WHAT A HUGE RELIEF!There remains two problems:
1) While most of the mutation stops have their own pipe rank - and therefore will work as described above, there is one stop in the swell and one in the choir that is a mutation pitch (whatever were the builders thinking!!?) The above method can't work with the above-described method. I have thought of two methods to cover this scenario.
a) have those two stops conveniently disappear - disguise the draw knobs or just make them blank; that's certainly possible because, as Martin described:
Note also that unified 'mutations' would only sound at approximately the same pitches as real mutation ranks, since a real mutation ranks is tuned perfectly to the harmonic, whereas with equal-tempered pipework there is no key-shift that you can apply to get exactly the same pitch from a non-mutation rank.
I've noticed that before. They are audibly ugly-sounding because of the tuning - AND, they can't be in tune because of this unification. In full organ, it's OK (I guess).
b) the second method that would include these out-of-tune pseudo ranks would be to wire the common rank - where pitches are borrowed - so that they can be played from both sources. The common rank would be played as first described but there would be a diode inserted in the circuit of each note so that current could not flow backward. The first method still works for this multiple use rank. Then, also have a separate MIDI driver board feed the (now offset in pitch that can be described in the CODM) pipe. It too would have to have a diode inserted as well so that its separate +VDC can't feed back into the source pipe wiring. In essence you would have two diodes connected together (via their cathode ends) and the result of that joining would feed the pipe magnet. The other end of one diode would be connected as described at the top of this message and the second diode would be connected to the offset MIDI driver to provide the mutation.
It would seem that this method will work with the addition of two smaller current capacity MIDI driver boards (only 1 note would be driven at a time - minimal current) - one for each of the two existing mutation pitches produced by unification.
As Martin also said:
If you drive the pipes at the 'division' (=wind-chest) level in that way then you wouldn't even need any ExternalRank or StopRank entries at all. You would just have a Stop entry for each real rank of pipes and use MIDI output from it directly to engage or disengage the real rank of pipes as a whole (as with a slider wind-chest).
and this is a BONUS!
You would still have to set up the two rogue ranks in the normal way though (as Martin previously described). It's all in the CODM manual.
Anyways, I've been away thinking of the best way to approach this and b) seems to be the best answer. Many thanks to NickNelson for sending my thought process in this direction! This solution is still < 1/3 the cost of the original idea of each pipe separately being controlled by a MIDI driver board. It seems that I've come a substantial way in 3 weeks from having known NOTHING about the mechanics and logistics of organ wiring, unification, and the like. It's easier playing the instrument rather than dissecting it on the mechanical/electrical level.
Thank you all for taking me just past the NOOB stage.