If you're willing to throw some money at the problem ($16), a Teensy might be just the ticket... Projects using the Teensy USB development board ...towards the bottom is "CD Emulation - A USB MassStorage Device that can emulate A CD Rom Library".
There's also this... http://elasticsheep.com/2010/04/teensy2-usb-mass-storage-with-an-sd-card/
And this... http://www.instructables.com/id/Music-Playing-Alarm-Clock/step6/USB-Mass-Storage-with-MicroSD/
For $8 you can add a backpack... PJRC Store