Looks promising to say the least!
It's really cheap too, and I would try it if I already had the maxplay adapter. Which I don't haha
I still had to think about your idea of soldering parts together. Maybe if you found a power bank with an output of those 15V required to power the Wii U (thus having more outputs than just USB ports) and the corresponding amperage to cover the consumption, I guess you could put a cable together that goes from the power bank output to the power socket of your Wii U directly.
I'd actually like to try that, but I don't know about such power bank, especially since most power banks support 16V rather than 15V and I can't say that I know what this means in practical use.