for use as AUX storage

At 115Kbits/sec, you wouldn't be able to play MP3s off it, even if you wrote a Linux block driver for it and somehow arranged for the player to mount it.

and or backing up their player?

Why not just use a PC, assuming you're not dead set on in-car backup? Not only are they quite capable serial-to-IDE adaptors, but they also function as (much faster) USB-to-IDE adaptors and Ethernet-to-IDE adaptors.

Or are you talking about completely subverting the serial port, and not running RS232 serial over it at all? In which case, you'd better wait until we come up with a way of upgrading player software which doesn't involve the serial port.

Peter