Thanks, I thought the same thing, so I went ahead and gave it a shot. Works great. As far as my PC is concerned, I have an XB360 controller plugged in.
I'm pretty close to dumping Retroarch though and going back to discrete emus. What a PITA that thing is.
Edit:
So basically just typing this for reference purposes, if someone comes along searching for this kind of info. The WiiNuSoft software gets the job done. (Research the difference b/n WiiNUPro and WiiNUSoft - the "Soft" is a lite version, and all you're likely to need.) The only downside to this is the controller itself - it's made for use on a Wii U, so there's no on/off switch. And once it's connected to your PC (Bluetooth), it stays connected even after you've turned off Retroarch and detached it in WiiNUSoft. So if you're putting it down for hours/days, that little blue light is going to stay lit (and your controller's battery is gonna drain) unless you also go into Windows' "Bluetooth & other devices" setting and disconnect the controller. And once you've done that, the next time you want to use it, you must start from scratch with WiiNUSoft. This is a 15 second procedure at most, but it's a hoop you have to jump through that might not be required with a paid solution (i.e. mayflash adapter).
Overall though, thumbs up.