Not necessarily. The goal is to have it working but I won't delay the initial release if it's not ready, so wiimote support may be added at a later time. Adding keyboard support would actually be a piece of cake compared to wiimotes.
Anyway, there are adapters that allows you to use various controls on a GC(port), such as a PSX controller. How well they work I have no idea about though. But they plug into the GC controller port.
Atm there is no way to use a USB/bluetooth controller or keyboard. You are limited to what accessories are available for the gamecube.
I have the EMS Classic Linker Plus & Mayflash Keyboard Converter 2in1.
So, I can confirm that they work on both the Wii & GameCube.
With that said...
Honestly, My favorite controller to play most games with is the GameCube WaveBird Controller.
I almost never use a keyboard mainly because not enough games support it & when I do use the linker it is to use my Wii arcade stick to play GameCube fighting games.