Here s my MMD specific FATlib called libFATwert. It is a (heavily) modified version of GBA_NDS_FAT by Chishm. It only has support ofr the Max Media Dock by Datel. It will not work with any other devices (This includes GnM [Games 'n' Music] !). I urge developers to use this to add support for MMD (You will have to make a seperate project). To use this library, use it in the same way you would use GBA_NDS_FAT.
Enjoy!
EDIT: The reason I didn't do DLDI is because this should work better than DLDI. For instance, Tales of Dagur doesn't work with the MMD DLDI. And DSO you have to rename it and it corrupts your files when you take it off. And actually DSO uses libFATDragon.