Well, the feat only applies vs mind affecting spells and abilities, I don't know you should fiddle with the base Will save.
If you want to anyway, I'd say make it an activation, and give it this code, around Post-Attr 10000:
doneif (field[hIsOn1].value = 0)
hero.child[vWill].field[Bonus].value -= hero.child[aWIS].field[aModBonus].value
hero.child[vWill].field[Bonus].value += hero.child[aCHA].field[aModBonus].value
As always, no assurances on the code, make sure you test and refine if necessary.
If you want to anyway, I'd say make it an activation, and give it this code, around Post-Attr 10000:
doneif (field[hIsOn1].value = 0)
hero.child[vWill].field[Bonus].value -= hero.child[aWIS].field[aModBonus].value
hero.child[vWill].field[Bonus].value += hero.child[aCHA].field[aModBonus].value
As always, no assurances on the code, make sure you test and refine if necessary.