The concept of controlling the organ from the MIDI source by teeing
into the keyframe tubing is not straightforward.

Complications arise as the keyframe, when the key is down, will vent
it's connecting tube to open air. This allows the pressure to escape
after the note, percussion or whatever has sounded from when the key
was last in the 'up' position.  (With the key in the 'up' position air,
under pressure, is sent from the keyframe)

To control the organ by teeing into the keyframe tubing would therefore
entail blocking off the keyframe side to prevent this venting the
tubing to air. It would be then possible to control the organ's
functions by introducing pressurized air via a relayed-controlled air
valve that is connected and controlled from the MIDI output.  This
still has not given any means to vent the system after the note (or
whatever) has finished operating and it starts getting complicated to
devise a 2-way changeover air valve that performs the keyframe
functions.

Hope this helps to answer the recent question asked on this subject.
                                        - regards, Ken Slow

(PS -   I would be interested of hearing if anyone has devised a good
way of controlling the organ by teeing into the tubing.)