So will the onboard interface allow for file transfer directly to a connected USB slave device, say a USB mass storage device (doubling as a basic flash-memory MP3 player; you know what I'm getting at) *without* a PC involved at all?

Although the hardware electronics in the Karma could (as described in that old thread) theoretically support something like that, it is not known whether or not the player firmware will be written to actually do it.

Keep in mind that, in order to do that, the player firmware would need drivers for the device in question and then the user interface would need to be written. I'm sure their hands are full just getting the thing working with a basic feature set right now.