It would be neat to have a code that just unlocked the 15 Japan Only Custom Parts. What I did was unlocked all the parts first before using the cheat.
I got a little idea on how GameCube Action Replay codes work when making one for Donkey Konga 2. You would first need to decrypt the codes, so we can see what the code is doing: http://doc.kodewerx.org/hacking_gcn.html
I'll have a look when I get some time.
EDIT:
Okay, this is what the US Code looks like to unlock the all the parts:
9C0030C8 00120000
840030C8 00239100
420030C8 0004FFFF
420030C8 0005FFFF
420030C8 0006FFFF
420030C8 0007FFFF
420030C8 000BFFFF
840030C8 FFDC6F00
00000000 40000000
What we need to edit are the "42" codes. So the first one is doing the following:
- Write FFFF to the following offset: 30C8 + (0004 * 2) = 30D0
What we need to do is edit the FF values. If you know your hex to binary, you would know that FF = 11111111
Think of each 1 that tells the game the part has been unlocked. So if I just want to unlock 1 part, I would put hex value 80 = 10000000
I need to play around with the RAM to figure out which values unlock which parts. Hopefully my emulator works.