Savegames, as well as almost all other files, are encrypted using some crypto magic. The keys were found and now there are some tools to decrypt and recrypt the savegames called Segher’s Wii.git.
To compile them, you need to also compile OpenSSL, add the include-directory of OpenSSL to the search path for gcc and also point the ld to the compiled libcrypto.a.
After that, find the 3 interesting keys on HackMii, which are md5-blanker, sd-iv and sd-key.
Create a directory ~/.wii and put the 3 keys in binary form in there. (No text file with the values as numbers and letters but binary files with exactly 16 Bytes per file. Use ghex2 or such.)
If everything is correct, you can uncompress savegames data.bin using tachtig and recompress them using twintig.