I found an answer to how it’s done on Wii a few years ago. Wii U I’m not sure about though.
The Wii does a render at a virtual res of 854x480 to an internal res of 640x480, then stretches that to 711x480, scanned out in the same 720x480 box as GC before your TV stretches it again out to the panel.
AKA super blurry no matter what you do, no clean pixels.