Mii Maker Miis
Now, I know what you're thinking...
You're probably thinking "Hey, wait a minute... Mii Maker doesn't have any Miis, it's Mii Maker!" And you'd actually be right... almost.
The Mii Maker icon has 6 Miis in it, most of which aren't regular Wii U/3DS CPU Miis. While this icon is just a flat image and itself doesn't contain any Miis, the developers left the Mii data files for these three in the files of Mii Maker. I found these six while looking through a NAND dump of my Wii U, and found them located in the folder \sys\title\00050010\1004a100\content\ffsd\ (though, if your Wii U is anything other than the USA region, it'll be slightly different).
When I initially saw the Mii Maker icon, I thought there wouldn't be any way we'd actually get official QR codes for these six Miis. But as luck would have it, we can!
For reference, here is the Wii U Mii Maker icon, extracted from the files: