Nice one!

When the mute line is activated, does it just mute the signal completely or just hush it to a certain level?

I've been looking at doing something similar, well actually it would probably fade the signal down quickly and then fade slowly back up, it would be completely separate to the empeg, in fact the empeg would be completely oblivious to what was going on.

I'll probably just build a ducker which is basically an audio compressor which works on the output chain but is keyed by a separate audio input (the sat nav). I can then tweak the attack and decay to suit.