IMC I allow an amulet of mighty fists to be enhanced just like a normal weapon. One can even use the Craft Magic Arms & Armor feat in place of Craft Wondrous Item. But since the monk probably already has a dozen items he wants to fill on his neck slot, I also allow other items to be similarly enhanced, namely gloves, rings, and bracers.
As most people who have any experience with monks in 3.5 will tell you, it isn't unbalancing at all. Most people with extensive 3.5 experience agree monks are underpowered relative to the other PH classes, at least insofar as their expected role, an observation that has already born evidence here. It's consensus at this point. In fact, it is still quite limiting to allow monks to only enhance their hands; meanwhile the fighter has a +5 cold iron longsword of chaotic outsider bane, a +5 adamantine longsword of construct bane, and a +5 silver longsword of undead bane he's ready to whip out of his bag of holding at a moment's notice (perhaps hyperbole, but not for all games).
I think allowing such enhancements to be placed on amulets, gloves, rings, and bracers is the perfect solution for myself and my games.