I'm 99% sure I've found a way to transfer the XD save to a GC memory card and having it work. However you'll have to start a new game. Requires this homebrew app.
1) Make a dump of the memory card you intend to use.
2) Use that dump file in Dolphin and start a new file in XD.
3) When you want to put the save back on the card use that app to restore the individual save (use Export GCI in Dolphin's Memory Card manager).
I've tested this with F-Zero GX, another game with protected saves (I hear it's more stringent than others but I have no proof to back that up). I dumped a freshly formatted MC, created a save in F-Zero GX in Dolphin, exported and restored the save, and the game didn't have a problem with it.
continued in next VM.
1) Make a dump of the memory card you intend to use.
2) Use that dump file in Dolphin and start a new file in XD.
3) When you want to put the save back on the card use that app to restore the individual save (use Export GCI in Dolphin's Memory Card manager).
I've tested this with F-Zero GX, another game with protected saves (I hear it's more stringent than others but I have no proof to back that up). I dumped a freshly formatted MC, created a save in F-Zero GX in Dolphin, exported and restored the save, and the game didn't have a problem with it.
continued in next VM.