I'm pretty sure you can't. If I remember right the code was made only for the sd slot. If you want to use a usb though you can try replacing the files on the iso and then play the game in usb loader. Just make sure you keep a backup of the iso so you can go back if you ever need to. You need an 8g flash drive to do it though.
Yes, me and my friend were wondering the same thing. As of what Miniova said, you would likely need an iso of brawl. HOWEVER: Rivolution may be able to load through a usb