I don't know Wii U specifics but assuming "5GHz" is the thing then "it depends".
Signal frequency is but one part of the puzzle of radio communications. Most antennae are tuned for a fairly small range, and most cards you plug into the PC are only going to play with specific signal types (far easier this way, see also why we have not got DS download play over normal network).
Sounds like there are existing chips that speak the right signal, though even if not you do have things like spectrum analysers (an expensive hobby, especially one that will do for this) and software defined radio. Software defined radio is a fairly fringe area of electronics, and might run into some kind of regulatory issues (though at the ranges you are likely concerned with then... legally I can't say won't get troubled but yeah). It would also be a lot of effort compared to putting a streaming option on a tablet and maybe wiring a bluetooth controller to that, said tablet would likely also be 1000x more capable than this.
If there are existing chips then you better hope they have documentation, else you get to reverse engineer those too (not usually the worst thing going and it will probably be USB which is not so bad either -- they will probably send audio and video and take controls back and there are limited ways of doing that). If there is documentation then someone will probably have made a basic example program for taking controls and spitting them out as presumably Windows compliant in some way (or at least some way you can feed into whatever the kids are using to allow all the various controller formats* for Windows to work on games not explicitly coded for that given controller type)
*Windows has an older DirectX take, a more modern one, think Valve has some of their own and there are a few others depending upon how you want to look at it, not to mention legacy stuff from long ago (gameports, gravis pads and such if you want a jumping off point) but that is probably a different discussion.
All of this is not necessarily on the basic side but still the sort of thing that we see in example projects for the likes of programmable chips -- see all manner of
https://www.pjrc.com/teensy/projects.html and things for arduino where they fake being a keyboard and attach to switches of their choice).
