IMO, if anyone is going to make custom hardware, they'd be better off building something with a decent SoC and keeping it TINY - smaller still than some of the others out there, with all the required bits to make auto connection simple (12v trigger relay input for instance). Then with integrated WiFi or Bluetooth, pair it up with an iPod Touch or iPhone. On the little computer you'd run Linux, Slim Center and the new player, and on the iPhone/iPod you'd run one of the controller programs such as iPeng. Hopefully with landscape support.

That said, you can probably do this now with existing micro PCs, though I'm not sure which ones would make for a nice glove-box install.