Bolts and arrows are allowed all on-hit cast spell properties. So all you have to do is modify the scripts (x2_s0_darkfire and x2_s0_flameweapon) checking the bolts and arrows inventory slots to see if they match the targeted item. (Refer to IPGetTargetedOrEquippedMeleeWeapon() in x2_inc_itemprop to see how it is currently handled).