Well, Pukachi accidentally removed the fog of war in FE5 when doing simple hex edits, so I didn't think that it would be too hard to end up doing the same here...
Removing FOW is easy if it's already programmed in. It's just 1 byte within chapter data that says if it's on or off (or how far you can see in the GBA case).
Adding it in when it isn't programmed is another story and would require ASM work. Since there is no data for it (let alone byte to change it on and off).