Mustrum_Ridcully
Legend
I am not sure, but I think Xahn`Tyrs interpretation seems to be right - as far as an interpretation can be wrong or right. 
Though in most parts Flurry of Blows works similar to Twoweaponfighting, there is actually no rule that tells us that it is in all things the same ...
Since most the time we do not try to adress certain body parts (no target zones), I think Magic Fang is meant not to affect a certain body part (a fist, as an example), but a certain natural weapon.
So, a creature having as natural attack options bite, claws and tail could have enchanted her bite, her claw or her tail with one Magic Fang.
Most Humanoids do only have one "natural" attack, the unarmed attack. Though it is only stated in the monks description, a unarmed attack can be more than just an attack with the fist.
I believe tripping with out a special weapon for it (like a Flail) is considered using an unarmed attack, and you may even try to trip enemies when you have both hand full with weapons and/or shield...
So I think if you cast Greater Magic Weapon on a Monk, he can fully profit from it.
This might also be the reason why the Amulet of Mighty Fists is much more expensive than a standard weapon enhancement...

Though in most parts Flurry of Blows works similar to Twoweaponfighting, there is actually no rule that tells us that it is in all things the same ...
Since most the time we do not try to adress certain body parts (no target zones), I think Magic Fang is meant not to affect a certain body part (a fist, as an example), but a certain natural weapon.
So, a creature having as natural attack options bite, claws and tail could have enchanted her bite, her claw or her tail with one Magic Fang.
Most Humanoids do only have one "natural" attack, the unarmed attack. Though it is only stated in the monks description, a unarmed attack can be more than just an attack with the fist.
I believe tripping with out a special weapon for it (like a Flail) is considered using an unarmed attack, and you may even try to trip enemies when you have both hand full with weapons and/or shield...
So I think if you cast Greater Magic Weapon on a Monk, he can fully profit from it.
This might also be the reason why the Amulet of Mighty Fists is much more expensive than a standard weapon enhancement...