I believe that the LUFA project should make this possible, but I can't figure out how to use it. I have an SD card attached directly to SPI using the SparkFun MicroSD breakout board. I can read and write to the card fine using the SD examples in the IDE. I have no idea how to get ANY of the LUFA projects working though. Can anyone help me get an *.ino to do this?
Just for Google-fu...
ATmega32U4 based Arduinos include:
Arduino Leonardo
Arduino Micro
Sparkfun Arduino Pro Micro