Yes, it's a great idea. Since I don't go in for monster stereo systems, I'd much rather have a simple box like a CD changer that just responds to my cheap factory-fit head unit.
Not surprisingly, you're not the only one to think of it - there was a suggestion just a couple of weeks ago on this board, and it seems to come up regularly on rec.audio.car.
Despite this, no-one actually seems to have succeeded. There was a posting on Usenet a while back from someone who'd interfaced his head unit, but it was an unusually simple one (an early alpine, I think). In most cases, there's a reasonably complicated serial control protocol that goes on down the changer cable, and the manufacturers like to protect their captive market (especially with factory-fitted units) and keep the details to themselves.
However, there is some hope : there are several companies that make interface units to adapt between different changer manufacturers, and although the one I talked to wasn't interested in making a generic mp3 interface that would suit me, someone else might. Maybe there's a changer interface that's really simple, like the alpine mentioned above, and you could get a converter to adapt your head to that protocol.
Secondly, although it needs some specialist equipment, it's not impossible to decode the controls. The manufacturers of those converters must have done it, and now there's an incentive for the rest of us to do it, too. I know nothing about the eclipse equipment you have, but I'm having a moderate amount of success cracking Clarion's C-bus : I've got it thinking there's a changer connected, and it will flip tracks on demand. Just a little more to implement and I might get the audio on !
So don't give up - dive in with 'scope, logic analyser and microcontroller and tell us all what you find .