i dont think this would be impossible and i dont think apple would stand in the way of it. apple would probably make the hardware and get money for it. they make car kits right? if they made the usb dongle or whatever its money for them.
the problem is will it make them enough money to make it viable to put into production. maybe a nice usb to stereo jack would work so any player (ipod, zune creative) could just plug in and audio is heard but channel would still have to be changed on the mp3 player.