I'll update when I get the new MicroSD card adapter. It doesn't make sense why it would be that as a MicroSD adapter is just a pin converter. I checked the pinout of a MicroSD adapter and I can confirm that DAT0, GND, CLK, and CMD are connected to the correct pads according to the guide on reboot.ms. At this point what
@Leeful said makes the most sense. The 3DS hard mod was picky as hell when it came to SD card readers. Maybe the Wii U is the same. It seems like not a lot of people have done this mod. I think
@Hikari06 has and
@lucasb (I think that was his name) has. Maybe they used a USB SD card reader.
I'd love it if you tried the mod yourself. Bear in mind that the probe points are quite small so you'll want a fine solder tip. I'm decent at soldering but I'm not an expert so I found the DAT0 and CMD points to be a little difficult. The CLK was easy peazy and the GND was not too bad it just required more heat. Go for it. The cost of entry is low if you already have the equipment. An adjustable temp solder station is probably not necessary but I wouldn't have wanted to try without one. Just don't use iFixit's teardown guide as it's next to worthless. Watch Jerry Rig Everything's video on YouTube instead.
Oh, and to run the wires out of the case I used the gap in the case halves that the front rubber tab thingy sticks through (on the right side of the console when horizontal).