So I have this on the way:

http://www.folcik.com/hi002.htm

Which will allow me to pull the faceplate off my G8 radio and replace it with a fascia and the remote display setup from my empeg.

But I want to keep factory BT and on-star. I have isolated the the audio signal from the BT/OS unit and confirmed that it works just fine when routed into (for example) the mic in on my laptop.

Can I feed it into the MIC in on the EMPEG and have it switch to that when the phone remote line goes hot? I know it (EMPEG) can mute, but will it pipe audio from the MIC?

I'd check the wiki but. um. yeah.

The empeg will not pipe audio out from the mic input.

You want the Aux input for that, not the mic input. I think there was even some work done on either the factory player software or the hijack kernel that would allow you to switch to Aux when the cellphone muteline was activated. Don't know how well that ended up working for those who used it, though.