Thanks Pete.
The reason I think this is possible is that the head unit :sm_TMI gets an input from the PCM to raise the volume relative to speed. I'm thinking this is where some external control could raise/lower volume with manual input.
I don't have any hope of having full control like the 10+s have for changing stations or songs.
Mustang World did something like this on an SN95, but they actually tapped into the Mach 460 head unit internals to accomplish it. They used the on/off steering wheel cruise control button for volume control and just wired a separate simple switch to handle the cruise on/off.
I'm no electronics guru, so really out of my comfort zone. But I do like problem-solving, and really motivated to get this done.