FYI, I had to send my second-gen MDT-X7000 (described in the worklog above) back to Xenarc for repair. One day, it stopped moving during extension, and never recovered (I'd been able to clear this condition before with a power cycle); Xenarc sent me an RMA in < 24 hours and I should have the unit back tomorrow (turnaround about 3 weeks).
I'm looking forward to seeing if the replacement has different software than the returned unit - anyone have advice on how to check the firmware version?
In the meantime, I've discovered that the Ridgeline subwoofer is powered by an amp that's integral to the OEM radio, so there's no good way to use the sub without the OEM unit unless I install another amp. I'm not game for that right now, so I'm going to try something different - back to the original design: PC->Xenarc->aux in on the OEM radio. I couldn't get this working on my first install, but it's worth trying again. The looks isn't as clean, but the OEM radio (sorry, I need radio here) is much better than the Xenie and it solves the sub and steering wheel control issues. Will post updates.