Give fiefs an option to build a blacksmith workshop using PP. It would enable repairing items for some amount of gold. Only items used in battle could be repaired, giving them +2. If item would be used in two battles earning -4 to its stats, repair would only take last battle into account giving it +2 back.
I think this should be too hard for devs to implement, if there would be a way to flag items that were used in battles.