From the PhB (which system? Both. Well, the CRB from Pathfinder, to be technical): "A monk’s unarmed strike is treated both as a manufactured weapon and a natural weapon for the purpose of spells and effects that enhance or improve either manufactured weapons or natural weapons."
Id, Craft Magic Arms and Armor: "The weapon, armor, or shield to be enhanced must be a masterwork item that you provide. Its cost is not included in the above cost."
Ultimate Magic (pathfinder only), Masterwork Transformation spell: "Target: one weapon..."
You can, and should, already do this. In 3.5 this requires that the GM rule that a Monk has a Masterwork body (pretty reasonable). In pathfinder this requires a level 2 spell, or GM discretion. I have a player who likes to play monks and this is usually something he puts a fair amount of money in. I have run into issues doing this with less RAW-friendly groups, as reforging one's hands into the Fists of Fire and Thunder can upset people who have got it in their heads somehow that monks don't get magic weapon upgrades. In fact, monks benefit from magic upgrades to their weaponry more than any other class, since they both can gain both natural-weapon-only benefits and manufactured-weapon-only benefits, and can have a great variety of specialized enchantments available pretty much all the time.
In both systems monks can make Unarmed Attacks with "either fist interchangeably... elbows, knees, and feet" (PHB, cf. CRB). Enchanting each of these body parts separately allows an unburdened monk access to what are effectively 8 different magic weapons. Her number of attacks doesn't change, but on each attack she can choose, as normal, which weapon she is employing. This allows monks to make better use of energy damage and bane abilities than most other melee classes. In campaigns where WBL is preserved by GM fiat the monk will never have the cash to do this, but in campaigns where spells and social abilities are allowed to generate their stated wealth without WBL limitations the party wizard should be able to provide the monk with a fairly kitted out set of enchantments around level 9.
Even with all this, however, the monk is still a spell-less melee fighter and so very underpowered. The monk will be much better than a fellow student who for some reason refused magical augmentation, and, in fact, a decent melee build if the game is limited to the Core classes, since the monk's class abilities help somewhat compensate for most non-casters' lack of battlefield mobility (i.e. teleportation), but the monk is still a weak class.
The benefit of doing your enchantments this way is, in order of importance 1) It's cooler. You can have Hands of Justice and Feet of Law. 2) You save a magic item slot. This is a big deal, as that slot can now be used to hold something like a Scarab of Protection or a Necklace of Netted Stars. You can also keep your Amulet of Mighty Fists +5 and use that in place of your enhancement bonus on your body parts, saving room for more special abilities 2) Your enchantments can't be stolen the way external gear can. It's always on your person and you can never permanently loose it without some very high level magic. 3) You can utilize all the defensive portions of the enchantments at once, and choose between the offensive portions as 'not an action' when making your attacks. 4) Technically, you save 300 gp. Actually, if 300 gp matters most of your benefits are moot.
You do not uniquely gain the ability to use weapon enhancements by doing this. You could do that already, via a Necklace of Natural Weapons or a Scorpion Kama, among other options.