Alright, I think I've managed to mod WUPinstaller to work. The source files crediar provides on github seem to be incomplete and was possibly using an older version of ksploit? There's no MAKE file, and the compiler throws a fit about his void** conditional that I don't know enough about C to debug myself, so I had to disable that part. Thus, use this at your own risk, as this has some safeguards removed that need to have their addresses fixed to stop throwing a fit on 5.4. I don't think it should cause any issues though.
It allowed me to install the Splatoon update and the system recognized it and allowed me to transfer it around. I will try SSBU next to see how it does. Have to go to bed right now though.
Due to certain things I did though, I can't be sure this fully works, so hopefully someone here is willing to test it on Splatoon. After you click the Wii U Menu button after it runs the second time, the screen should freeze and "Internet Browser" should popup near the bottom. Let it sit that way for a good hour and then reset your console.
edit: Oh, also, make sure to delete the existing failed update data or corrupt data if you have any before doing it.