Hi everyone, I'll explain my need.
I would like to take advantage of the sounds of the Model D app (on Ipad) through the keyboard of my modx6 (which I should split in two zones). I already use the iPad to send the program changes to the modx and to read pdf scores (via Forscore).
Now I would like to use the modx so that I can play the app's Moog sounds, but I would like to make sure that Model D is activated on the song in which I must play solo, while on other songs it is deactivated. Is something like this possible?

It is possible to treat the app as a PART within the Motion Control Synthesis Engine by routing the audio from the iPad to the MODX A/D Input.

You need to connect the MODX USB “TO HOST” port to your iPad with a USB cable and Lightning-to-Usb-Camera-Adapter.
Connect the iPad’s 1/8” Stereo mini headphone output to the MODX A/D Input using an appropriate adapter to deliver the signal (stereo 1/8” mini male to two 1/4” mono male phone plugs). I actually have a stereo 1/8” mini male to two 1/4” phone female adapter from which I use standard 1/4” cables... (stereo 1/8” mini male to two 1/4” male phone cables exist in specific lengths)

Normal configuration you would not have to include this cable/adapter to the A/D Input — meaning: you could connect just via USB and the app would return audio via the digital connection direct to the MODX main outs, but you would not be able to manipulate the apps audio further within the Motion Control Synthesis Engine...

By delivering the iPad audio Out to the MODX via the A/D Input allows you to control that input from the iPad as an external audio source, which means you can treat it as one of the 8 KBD CTRL Parts.

The MODX can control 8 Parts, simultaneously. They can be a combination of Internal Parts and External Zones. You can assign the A/D In its own two Insertion Effects — and you can control the parameters of these effects with the Super Knob/Assign Knob system — you can set a Part/Zone block to control the external synth. This allows you setup Note Range, Transpose, Volume and Pan control, etc...

This integration follows this signal path: set up a MODX Zone Part slot to transmit to the app. That zone slot would have IntSw set to Off so that it will only transmit Out to the app (select any Midi channel not being used by the MODX). The Zone slot gives you access to Channel, Note Range, Transpose, Octave, Volume, Pan, etc... to communicate with the MiniMoog app. The returning audio from the app is routed to the A/D Input where we can then apply two MODX Insertion Effects (w/real time control), a send to each of System Effects and as with all signals access to the Master Effect.

Hope that helps.

It is possible to treat the app as a PART within the Motion Control Synthesis Engine by routing the audio from the iPad to the MODX A/D Input.

Isn't it also possible for them to use the Digital In and get the audio over USB? An AUv3 host like Cubasis or AUM might help with any necessary audio settings on the iPad end, if the audio isn't automatically routed correctly over USB by iOS or Garageband.

(The Moog Model D app is able to run as an AUv3 plugin, and is also now free by the way in case anyone hadn't heard.)

Besides saving on cables, that would eliminate any potential D/A to A/D noise for the iPads that have a headphone jack and it would save the day for the new ones that don't. The newer iPad Pros require a USB C to USB Type B adapter or cable (a "USB C printer cable" as I see it called sometimes, photo attached) rather than a camera kit, by the way, as they don't have a lightning port.

Edit: Oops, I'm sorry, I somehow missed the bit in the middle where you explained your reasoning for preferring the A/D input. In that case, if you have an iPad without headphone jack and you want to run it through the A/D input, a separate USB audio interface would need to be connected to iPad in order to obtain analog output, as well as a USB hub to also connect the MODX to the iPad simultaneously.