This should be out there somewhere...but in case anyone's looking to do this...I can describe FFX's weapon customization system in fairly deep detail.
In Final Fantasy X, weapons have special abilities and some actually have extra slots. Now, if a weapon has certain "abilities", the weapon is actually changed (Example: If you give Firestrike to a basic Longsword, it will become a Flametongue).
This customization is done through a menu and different abilities require different reagents to acquire. Example: Firestrike requires a few Bomb Cores (5 if I remember correctly). Weapon's CANNOT regain slots that are used. These "abilities" can modify everything from the weapon's element and capabilities (Break Damage Limit, Double-Overdrive, etc.), but also the character's stats (+20% HP being one I've abused for my armors and strength modifiers for Auron's weapons), and even some external abilities like increased encounters, no encounters, or the ability to "capture" enemies to battle against in the Arena.
The Customize option is added to the Main Menu when Rikku joins your party in FFX, just a little miscellaneous factoid.
So for the script, this is what would be needed:
A way to "store" these modified weapons (I think Blizzy made a custom weapon script that LITERALLY makes weapons from scrap (look to that I guess, because I think that thing uses armor IDs starting at 1000 rather than the pre-inscribed maximum value of 999), defining what weapons can have how many slots (some shops may sell the same weapon, but the weapon may randomly have a certain ability or extra slots for customization...believe me, when I found my 4 Slot Longsword in FFX and turned it into something that was only replaced by the fully-upgraded Calladbolg, Tidus' God Weapon), and a menu to manage the customization of the weapon.