It would be really nice if there could be a callback when an item is equipped (wielded) although I realize this could cause a lot of clatter using mouse wheels etc.
I have actually done this with my visible wielded item mods by crudely polling the current wielded item, there is a slight delay but it does solve the afore mentioned problem.
I just think it would be nice if there could be a cleaner implementation of this, either provided by the engine or the built-in scripts.