Ok I feel like I’m getting closer after a bit of Motu Forum hopping and with your suggestion above to look into Audio Midi Setup.
So apparently the Motu has a ‘virtual’ stereo pair of inputs called Return 1/Return 2. I think the goal is to get system audio to route to this pair and for this pair to show up in CueMix (currently they don’t).
Here’s a screenshot of the tools I’m fiddling with. You can see the Returns show up both in audio Midi Setup and in the Motu’s Channel names, but not in CueMix: