Book of Exalted Deeds
In it, there are 2 ways to advance weapons,
1 : The Ancestral Relic feat, where you do a ritual and expend the gold pieces, your weapon advances, it just exaclty like paying for the bonus, but if you dont have acces to smeone who can actually do the magic on the weapon
2: Anointed Knight (presitge class), at certain levels you can anoit your weapon with an oil you make and it gains certain properties, you can also anoit yourself
There is also the Kensai prestige class, Complete Warrior, where you sacrifce xp to enhance your weapon permanantly.
What you can do, is use a mix of these.
Firstly, base the maximum bonus of any weapon, on what a cleric of thier level could enhance a magic item using "Greater Magic Weapon".
Secondly, to mix the oil, they must gather the right materials for the effect they wish to produce (the materials could be a certain herb, skin of a certain creature etc, best way to do this, for a magic effect on an item, theres usually a corresponding spell, check thats spell components to find something suitable) - The materials, or time spent gathering them, should roughly equate to the gold value of the bonus, its just a little more depth to magicing up a weapon.
Third, anyone of say level 3 or above can create this oil, its a feature of the campaign, magic items are created by thier owners, and are tuned to them, putting the oil on is a sacred ritual, a way of giving praise to thier God, and thier god in turn blesses the weapon with divine power, so every magic weapon is blessed (how bless works depends on the gods alignment)
Fourth, any magic item found will be +1 and its powers can only be unlocked once you discover the weapons diety, and perform a quest in that gods name
That might give your weapons more flavour, doesnt hurt to have an xp cost each time the oil is applied either, soay 10% of what the gold value would be (I think thats how the Kensai does it) and make sure they can only mix the oil maximum of once per level, maybe every 3rd level (like the clerics advancement on the Greater Magic Weapon spell). You can also apply this to armour just as easily
Feegle Out
