So, does it actually allow the head unit to control the iPod...? It doesn't say that. Or does is just pass through the power for charging and the audio back to the head unit. For $10 I guess it's just the wires and the connectors - so you choose the tracks/playlists on the iPod and then select the CD changer on the headunit.
Apart from the fact that you need the reverse operation, there is the small matter of actually being able to control a CD changer - selecting discs by number (1-6), selecting next/previous, etc, not to mention the problem areas of start up and shut down.
As I said before - to provide control to a CD changer interface requires hardware and software. Whatever is used must have a USB slave port that will connect to the head unit. Some software to handle all of the complex communication that goes with USB device enumeration, then some buffering and intelligence to translate the commands, reply where appropriate, translate and forward where appropriate, then more hardware to convert to the interface of the CD changer. I am not saying it can't be done. Just that it is very complex, and not a quick hobby project.