Not sure what level we are playing at here but I will be assuming you can decode the files from the iso into a "plaintext" format.
Wii standards vary a bit from older systems owing to increased memory capabilities and the like, still many are very similar to the GC stuff which has a fair few documented formats:
Chapter 15. http://hitmen.c02.at/files/yagcd/index.html
Many games use "SDK" formats (in fact of those that I have pulled apart or others have documented it is a surprisingly high number compared to other systems past and present) so you can infer some stuff from those projects. This path is very perilous though- witness much of the DS, early DS stuff used a lot of SDK like data which then shifted later in the life of the DS leaving several people in an awkward position.
An easy way to see what I am talking about would be to look at the DS formats: http://tahaxan.arcnor.com/index.php?option...=36&lang=en
See how most will have a magic string identifier, file length, header data and so on in approximately the same order.
Compression will likely be an issue on the wii, most is a variant of LZSS, RLE, huffman or one of the many other "common" formats.
I might also suggest looking into the DS as hacking there is far more developed that the wii.
After that it is relatively simple coding, just pick a language that supports binary manipulation easily as not too much can be done with "known" data structure languages.