Drop your SD file structure onto your USB device. It'll load when SD is not inserted. if you remove SD while HBC is active it'll try to load USB anyway.
Everything, see above. Includes, homebrew apps(that's your boot.dols), triiforce, media files, covers for USB loading.
The only thing I don't think we can use USB for is actually running bannerbomb to get the thing started because it's launched for the SD channel/Data Management.