Followup, I figured out you can manipulate the end total directly, just not the way that makes the most sense:
Post-levels/10000 timing:
Code:
doneif (field[gIsEquip].value = 0)
doneif (field[gIsAttuned].value = 0)
~ add 1 to total proficiency bonus.
hero.child[ProfBonus].field[tProfBonus].value += 1
The first two lines check to see if the item is equipped and attuned, then it adds 1 to the total bonuses added to proficiency.