The point is every button press is being read as HID when using any controller/adapter. It's impossible to implement without making it read as standard gamecube ports (massive code change, waste of time)How could it respond in HIDTest is the GameCube game is responsible for sending data through the cable? Is there a GBA game that can send data to the GameCube without the need for a game?
Edit: I just remembered using native ports on the Wii disables bluetooth and HID controllers. Pretty sure it's impossible to implement the adapter as a new controller type just for GBA support.