Nymph race has similar ability. It's hooked in race_skin.nss script:
if(GetHasFeat(FEAT_UNEARTHLY_GRACE))
{
int nGrace = GetAbilityModifier(ABILITY_CHARISMA, oPC);
SetCompositeBonus(oSkin, "UnearthlyGraceAC", nGrace, ITEM_PROPERTY_AC_BONUS);
SetCompositeBonus(oSkin, "UnearthlyGraceSave", nGrace, ITEM_PROPERTY_SAVING_THROW_BONUS, SAVING_THROW_ALL);
}