I ended up with a PIE (Precision Instrument Electronics) MPSS-4. Turns out, it completely cuts the power to the secondary device when signal comes from the primary. It leaves the primary on at all times though.

Well, it seems that you could change it so that the secondary device is connected directly to the power instead of to this device, and the line that's currently cutting off its power cuold be used to control a relay inserted in the audio output line. Maybe? Just an idea.

Also, I'm thinking about a new car and thought I might be interested in a S60R (if and when they become available) or an S60 T5. Looks like yours is a much different model, but what is it, and what lengths did you go to to do the install? I think that the S60's dash looks even harder to deal with.