There's software that can be used to patch the ISO themselves, its what I use for my Wii games on my Wii U
I believe this is what i used, though its been a little while.
https://github.com/JoshuaMKW/GeckoLoader
I know there are gecko codes to enable 60fps for MK Wii in 4 player mode. I wonder if overclocking helps the performance on a patched 60fps MK Wii game
I just realised there's an option in Settings>Input>Allow Wii U Home Menu disabling this will allow you to open only the Retroarch Menu with the Home Button :D (don't forget to save RA cfg after) ;)
I'm in the process of updating the our current...