There is a good review of the Sony Ericsson HCB-30 hands free bluetooth car kit here.
It has a mute line that would be easy to connect, but is there a way to integrate the speaker output through the empeg instead of using the supplied speaker?

heh... i'd like to know this as well since i plan on getting it for my new phone. Haha... funny that that reviewer is installing it in a WRX too.

Piece of Cake Take the Raw Speaker level output and Attach it to "line level Adapter" Radio Shack Most stereo stores and Cruschfield all have them for 20 bucks or so .. Converts Speaker level output to rca then you could plug it into the Aux input on the Empeg..

The Adapter is nothing more then a antinuator(sp) you might also be able to get one to plug into the Mic Input also.

This actually seems pretty easy (here is a related post.)

Apparently Hijack can automatically switch to the Aux input when the phone mute line is active. This begs the question, when the "mute" line is active, doesn't it effectively mute the empeg sound output? Doesn't this carry over to the Aux line? Or does it just mute the current audio source, switch to Aux and restore full volume?

This begs the question, when the "mute" line is active, doesn't it effectively mute the empeg sound output?

Not if you program the software to do otherwise. The mute line is completely controlled in software. I think that you tell emplode to "do nothing" when the mute line changes, and instead have Hijack switch to the aux input.