With mocha 0.3b, you can use FAT32 HDD as if it was a WiiU formated HDD (the console will not know), but the game will be very slow as it's using nintendo's FAT32 drivers to access the game. nintendo did a bad FAT32 driver (maybe on purpose).
So, using 0.3b, you can store your wii and gc games in one folder, and your Wiiu "fake, redirected USB HDD" in another folder.
It's also a beta and it's missing lot of features, like copy/move/install game directly to USB from wup installer.
you'll have to do lot of tricks and edit manually the files on the drive to make the game work.
not sure it's worth your time.