I'd really like to be able to 'dd' the drive to other storage as a backup occasionally...

IIRC, there's some kind of out-of-band commands you have to send it to mount/unmount the drive via the USB mass storage. Effectively, you're telling the player to relinquish control of the drive to the USB chip and vice versa. This would require more effort than just adding the manufacturer/product IDs to a header file.

Moreover, once you have connected to it as a mass storage device, the Karma uses a proprietary filesystem, so you won't be able to mount it directly.

Assuming that you can jump the first hurdle, you ought to be able to use 'dd' for backing it up, however.