Alternatively, could use the OnDamaged event to make the player instantly apply extra damage if they possess sneaks and the monster is immune. Main catch is something like Damage Reduction or Damage Resistance would impact twice and if you made the bonus damage something like Magic to bypass that then something with 50% physical immunity wouldn't reduce the magic damage.
So if you only have Immunity, can just deal extra damage.
If you only have resistance, can make it a resist type people don't possess.
If you have both, you're kind of hosed.