Proof of concept Wii U Gamecube adapter plugin for Project64.
Latest release can be found here.
Copy the .dll to the Plugin directory, then select the plugin in the emulator.
If not done already, you will need to install the WinUSB driver with Zadig. If you have a third party adapter, you need to switch it to Wii U mode. See this page for more detailed instructions.
Use MSYS2 with the following packages installed:
mingw-w64-i686-gccmingw-w64-i686-libusb
Run build.sh to compile.