Hello Jeff,  I don't usually come on negatively to interesting
projects, but my own feeling about adding MIDI to a tone-wheel Hammond
organ is that you could save yourself a lifetime of effort and a ruined
organ by forgetting about it.  (You may consider this a challenge, but
then you may have a lot more energy than I, too.)

Each Hammond organ key operates something like nine individual
contacts, each serving a particular overtone of the fundamental pitch.
The drawbars control the intensity of each of these overtones in order
to create the voices.

There was an Aeolian Hammond player organ made in the past and it had
a set of pneumatics which physically operated the individual keys in
order to play the instrument, very much like a player piano would.
Registration and loudness was managed manually by the person
supervising its playing.

On the recording side, I have heard that some artists use the contact
for the highest overtone to operate as a MIDI switch in order to
communicate with other MIDI instruments or to record sequences.  This
affects the sound of the organ somewhat due to the missing partial.
You have to be really adept with a soldering iron and pretty steady
with your hands, as well as know where a lot of tiny little wires go,
before you should venture here.

If you really must play from MIDI, I suggest some solenoid or pneumatic
arrangement that physically activates the keys and the fixed available
"stops" or voices.  Volume control may have to be physically attained
as well, as the swell pedal (if you really want to call it that)
operates by a capacitive link into highly-sensitive preamplifier
circuits prone to lots of hum if you get it wrong.  Of course, you
could also vary the volume more easily at the power amplifier level
after everything else.

There are, of course, a number of MIDI sound modules that imitate the
Hammond and if it is the sound you are looking for more than the fun of
making an actual instrument play, this is undoubtedly the way to go.
The Hammond sound modules are getting pretty good.

Best of all, learn to play the damned thing.  It can be a barrel of
fun!

Jim Heyworth