What's "Flatmii 125"?
Anyways, everything that is connected via flatcable but does NOT actually use the dvd drive will work in all known Wiis, including the backup proof drives. If it has an additional function as working as drive chip, then this feature won't work on the new drives, but the others should work.
You have to know that these flatcable mods emulate a complete drive and should even work if you would take the drive out completely.