Simply renaming the file doesn't work. The game I try simply asks to create new sav data again.

Oh and I did a full backup of the memory card in RAW format and replaced the RAW file Dolphin used. Now I got my GC saves working in Dolphin.

Just need them to work for my games on the real GC since most games don't want to load/save gamesaves on slot B where the memory card currently is.
My homebrew launch method is the Action Replay exploit with SD Memory card adapter. So the adapter is on slot A. I can't bot with it on slot B since Action Replay will refuse to do anything unless it's got a memory card to save to in slot B. Sure I could probably do some kind of hotswap trick. But I would like to avoid doing that.

EDIT: Using the mdsc.exe file that comes with GCMM doesn't work for me either. I convert a GCI to SAV file and it's the wrong filesize. Game boots up and asks to create new save file like before, so the files it makes don't work.
