Okay...here's what Ive come up with.
Vorpal Missile
This enchantment allows an arrow or crossbow bolt to sever an opponent's limb or even a head. Creatures with no permanent shape or no visible limbs are not affected, such as oozes and many abominations. Most other creatures, however, are affected and may die if their heads are severed. The DM may have to make judgment calls about this attack’s effect and what limb was hit, if any, or if the player makes a called shot at a limb. A vorpal missile weapon can only be an arrow or bolt.
When hit, the target must make a Fortitude save with a DC of (10 + Damage dealt) or have the limb hit severed. If the attack hits center mass (torso), the vorpal enchantment doesn't work.
Caster Level: 18th; Prerequisites: Craft Magic Arms and Armor, keen edge, death spell; Market Price: +4 or 5 bonus (??).