Guide created by Andrew Lee
Note that this guide does not go into detail, it's just providing the commands to install Windows.
Be cautions when doing this when dualbooting, please backup any existing data or you will lose them all.
First open CMD by pressing the following keys after booting into setup: Shift + F10
notice: Wii U linux is not production ready! Proceed at your own risk. Written on 2/3/18
Quarktheawesome and rw-r-r_0644 have released an early version of Linux for Wii U!
The PowerPC side of the Wii U requires a custom linux kernel, which can be found here. The ARM side of the Wii U requires a custom fw.img file to load that kernel, which can be found here.
Once the kernel is up and running, the first repo is configured to automatically try to load programs from /dev/sda1 in an inserted usb. The first partition of an ext4 USB stick should contain a root filesystem for a linux distribution. See here for more information on a prebuilt PowerPC image