Have you ever used a raspberry pi? a slow processor in a linux-based OS is slow yes, but it's not quite as terrible as you may think.
Also, I wonder if looking at the Wii eShop games on Wii U will help. They run "natively" on the Wii U in a wrapper of some sorts, but don't have access to upscaling/extra Dolphin features. But they aren't running the Wii U in vWii mode from what I can tell, so if you're able to look at the game files and reverse engineer it you may be able to understand how the Wii U translates Wii instructions.
Also, I wonder if looking at the Wii eShop games on Wii U will help. They run "natively" on the Wii U in a wrapper of some sorts, but don't have access to upscaling/extra Dolphin features. But they aren't running the Wii U in vWii mode from what I can tell, so if you're able to look at the game files and reverse engineer it you may be able to understand how the Wii U translates Wii instructions.