A little experimenting wound up with some revelations that I think warranted a more generalized post, but as an update, I think the way forward for a true "audio volume pedal” is actually to use an atomic instance of MG3 with a simplified preset.
In a nutshell, this preset has MG3 take the expression pedal’s raw input, remap it to a nonlinear curve like MG3 can do with its Patchbay nodes, and then send that nonlinear output to the Gain plugin on an audio Chain. (Gain seemed smoother than connecting to the Mixer.) I had to tweak things a bit (e.g. configure the curve to not go above unity gain, find the right shape for a good volume swell sound, etc.) but it’s by far the best solution I’ve seen so far, and I think worth further experimentation.
And that’s where things got more generalized. I remembered–duh–that I might be able to take that remapped curve, and instead of sending to to Gain internal to the MG3 preset, send downstream as outbound MIDI–and therefore making it available to other elements in AUM. That discussion is elaborated further at the other post, but at least for now I think I have an acceptable “volume pedal” solution to go with! 