discovered that the head unit monitors the voltage ripple on the 12v supply line to determine how quickly the alternator is turning.

The empeg doesn't have any way to monitor the voltage ripple on the 12v supply line that closely (as far as I know). However, it has lots of other inputs which could be used for something similar. For instance, a small interface board could grab the current speed from the OBD-2 computer in the car and feed that to the empeg through the serial port.

Something else to consider is that the player has a microphone input. I think using this microphone input would be a much better way of adjusting volume on the player, because you could adjust based on actual road noise instead of speed. My last set of comments on this subject are here, where I outline why I think this would be a better way of doing it. It would require that you mount a small cheap microphone somewhere outside the car (such as in a wheel well, or behind a bumper, or some such) so that the music playing on the interior of the cabin didn't affect the sampling. The beauty of this would be that it could be done entirely in the Hijack software, there would not be any interface boards needed. So it's just a coding project. (The user could supply his own mic hardware. I've got just the one, in fact, it's a really tiny cheap little mic that came with a really old sound card. Smaller than my thumb, I could mount it anywhere on the vehicle I wanted.)

In other words, what you're talking about is do-able, it's just that no one has done it yet. I think that the main problem with this project (and with a few other projects like it) is that the people with the ability to do the work aren't the same people as the ones who want the feature for themselves.

In order for a "pet project" like this to get off the ground, you need to have someone in which the two "sets" intersect: Ability and desire. Brian made backlit buttons because he wanted them for himself. Jan made GPSapp because he wanted to get route directions on the empeg screen. Mark made Hijack because... well... I don't know why Mark made Hijack (but I'm really glad he did).