When I stealth-mounted my radar detector, I briefly considered doing this.

Then I realized that I didn't want the two systems (radar detector and empeg) to be in any way dependent on each other. Very important to keep them separate, IMHO.

As far as getting data out of the detector, that's highly unlikely with the cheap ones. The one I hacked up barely had the speaker output and the LED display. It was by no means a high tech device with external communication circuits.

I wouldn't be surprised, though, if wiring the empeg mute line to a relay connected to the radar detector's speaker wire, would result in a nice muting of the empeg when the detector went off. However I'm not sure you'd want this. My detector startles me enough when it goes off, I would hate for the muting of the empeg to cause it to startle me even more than it already does.