Extracting the files out of the game is one thing.
Getting the Wii to use your modified files is a whole 'nother thing. Consider for a moment that the Wii reads almost all the game data off the read-only disc. The only writeable area is the save-game on the internal 512MB, but unless there was a way to embed modified game code/textures/objects/data into the save-game data file and get the Wii to then use it and override what's on the disc, I think you'd be stuck. And I can't imagine a game written to allow that much power from the save game data file. More like, just info like location, stats, points, inventory, tasks accomplished, etc.