This is what I was thinking about doing, maybe it will give you an idea.
First install a monitor switcher that would let me go back and forth between two sources on the stock screen, that way you would still maintain a stock look. Here is the one I was thinking of.
http://www.sonicelectronix.com....html
Then get something like this
http://mobile.jvc.com/product....age=3
I have an O4 so I was thinking of mounting it where the CD changer is in the center and the changer in the glove box or ditching the changer all together. Most head units have very in depth remotes now days so you could get creative on where you would want to mount it.
Then I would patch the audio into the tape player using the mod Boon has outlined.
Just an idea, as was said before given enough money you can do anything you want.
I liked this idea cuz it seamed to give me every thing I wanted as far as nerdy tech features, it was not intrusive into the looks of the center section, and considering how cheap single DIN head units are now days it didnt break the bank.
Keep us posted.