I friend of mine has a Blaupunkt unit, this unit has a microphone that monitors the 'noise' in the car. If you rev the engine up, the stereo does too.
The Mk II has a mic nput, so it should just be a software issue.
For the Mk I, I think there's an external solution. Conrad Electronics used to sell a simple and small do-it-yourself kit with a microphone that monitored ambient noise and increased or decreased the audio signal accordingly. It operated on line level voltage and current, so it could easily be inserted between empeg and amplifier. It also was rather inexpensive. I don't know if it's still available though.