@Nintendo Maniac:
Why don't you test Linux partition then?
I don't have linux, or other OS to test. But I guess it only matter what the partition type ID is (testing all ID one by one).
USBLoader's open source nature : ok, it's possible to add it, if the partition format is open source too. USBloader is not the only one needed to be opensource.
Computer: how do you think you will make windows read unknown, not opensource, partition format, to add files to it? you need a manager, much like WiiBackupManager? drivers? For example, Windows XP doesn't work with GPT HDD at all.
I'm 90% sure that the WiiU don't care what the partition type you are using are.
it only check for the presence or a partition table.
If it finds a table it asks to format the drive in WiiU format without a table. (table can be at the start or the end of the HDD)
If it doesn't find a table, it assume it's already in WiiU format and doesn't ask to format. (like he tested above, having FAT32 partition without a table, WiiU didn't ask)
You can create a partition (FAT32/NTFS/WhateverLinux/OSX/any other hundred of types) without a table partition. This could work, I think nobody tested that.
Some USBloader have difficulty to mount these HDD without a table, but some USBflashstick are working like that. One big partition without a table (they usually don't need to be partitioned).
If you want a tool for computer to see a partitioned WiiU drive, it's possible and explained in the link I gave above, by faking the HDD size for WiiU and temporarily un-faking it on computer.