So, there are many good reasons to emulate a keyboard device. 99.995% of security programs do not restrict them is probably chief among them.
One issue with emulating the keyboard device, is that you can't write to it. To write, you need to be a remote storage device, which IS protected (in some cases.)
However, you CAN write to a keyboard device. That's how you turn on the keyboard caps lock light!
(old article that should still be valid, http://www.beyondlogic.org/keyboard/keybrd.htm)
God only knows how difficult it would be to make an program that transferred data this way, and I bet it wouldn't be too speedy, but there is no reason it couldn't be done, and the transfer would be at USB speeds, even if it was serial. It could be used to copy smaller files, or copy data over time (plug it in to the back and leave it overnight.)
Any thoughts on this being possible, or likely to be done in my lifetime?