Alternate clothing bonuses suggestion:
While you are adverse to clothing being more then vanity items for the most part - some clothing is required to start and/or complete some tasks, like the waiter / waitress uniform being required to work at the diner - how about some sets of clothing give a minor bonus to an out of combat event ?
Examples:
Waiter / Waitress set gives a 10% bonus to tips earned whole working.
An Exercise themed set gives a 10% bonus to working out at the gym.
The Succubus set gives a 10% bonus to practicing magic in the tower.
A fishing themed set gives a 10% bonus to catching bigger fish.
Works something like the indecent and naked status effects but requires wearing a specific set of clothing and being in a specific area to get the bonus.