FWIW: 'data.bin' format has been figured out, and Virus has mentioned he may support it in future.
BUT I've tried both, I don't see any reason why you'd want to use the Wii data.bin method instead of the gamesave extractor (assuming you have a game disc)

> I find (save files that have their save flag removed) a lot more convenient than using save extractor for some reason
Really? Am I missing something?:

From main menu, with Save Extractor
#1) Start HBC (2clicks), #2) Start Extract (2clicks), #3) Run it and exit (2 button presses), done

From main menu, using the Wii save method
#1) Start Wii utilities gamesave (4 clicks), #2) delete old gamesave from SD (4 clicks), #3) Save to SD card (3 clicks), #4) go back to main menu (4clicks, or home)

Also I find the actual writing to SD card to be much slower (even though it is writing approximately the same number of bytes). The reverse is just as bad.
The downside is the Game Extractor needs the game disc in the drive. All legal owners should have that, and I assume you just ran the game.
What am I missing?