The part tuning seems to be controlled in a MIDI by an RPN (Registered Parameter Number) at address MSB:00H LSB:01H
See 'Fine Tune' in the table at the top of page 98 of the Genos Data List for details.
You can edit this setting in MixMaster but the program only seems to allow you to do this by using a 'slider', which is difficult to set accurately.
MixMaster shows the values in what I assume is decimal notation (?) with 0 being -100 cents, and 16256 representing +100 cents.
I did a simple test recording on Genos with the voice Tune (Fine Tune) for channel 1 set to 0, channel 2 set to -64 (= -100 cents) and channel 3 set to +64 (= +100 cents). After saving, the resulting voices on those channels were detuned by -1 and +1 semitone as expected.
Using MixMaster I changed the Fine Tune RPN values for channels 2 and 3 and when played back on Genos, they were changed in line with the modifications made in MixMaster.
I then looked at the original song in Genos' Song Creator, Step Edit.
The Fine tune events appear as follows:
Ctrl 101 value 0 = RPN MSB
Ctrl 100 value
1 = RPN LSB
(value was wrongly shown as 0 in original post, corrected by edit to post)! 
Ctrl 6 value 0-127 = Data MSB (value 0 = -100 cents, 127 = +100 cents - set this to give required tuning).
Ctrl 38 value 0 = Data LSB
(Note that Song creator does not seem to add these events for channels that were recorded with 'normal' tuning, but a Data Value of 64 resets back to 'normal' if you want to reset a detuned channel).
I was able to edit the Ctrl 6 data value as required with the expected results, and even add multiple tunings throughout a channel by adding the full 4 event sequence at the relevant points.
This is not something that I am likely to want to do, so I'll leave it to you to work out the 'finer points'!

Regards
Ian