DIOS MIOS is only a way to redirect the disc access to another device (SD or USB), load an iso or an extracted filesystem of the game.
the Wii is launched in gamecube mode and there's no "on the fly" texture replacement.
If you want to replace textures, you need to do it manually, replacing the files in the extracted game format. But I suppose not all pictures are in their own files, there are certainly packages of multiple textures in a single big file, so you will have to find a way to create such package to replace them.
And you will certainly be limited by the memory of the console (you can't add high-res textures, you are limited to the same size)