This would require someone porting Zandronum to the Wii which would be very difficult and time consuming, I don't think this will ever happen.
Does the software renderer for Zandronum depend on x86 processor assembly language? If so, then a whole new renderer would have to be written.
OpenGL is out of the question, unless one wants to rewrite it from scratch for the Gamecube/Wii GPU.
Other than that, there is a lot of OS bindings in Zandronum that would have to be rebound to a Gamecube/Wii SDK.