A quick but revealing post from Desslock at the QuarterToThree forum:
Can you aim for specific appendages outside of VATS, or what?
Yes, and you can inflict the same damaging effects (like hitting an ant’s antennae to confuse/enrage it and cause it to attack its buddies). It’s just much easier to do through VATS mode, since you have a greater chance to hit with a shot aimed through VATS.
Still no comments on the impossibility of shooting a rat in the groin.