The RAW says that leg and torso hits count as misses if the target is in Partial Cover. I just had a situation in which an enemy Marauder assumed Partial Cover around the corner of a building and aimed an RPG-16. A PC fired at him and got a "Leg" hit. Would you be able to aim a rocket launcher around the corner of a building without exposing at least one leg?
[EDIT]
Just thinking out loud, maybe one way to handle this would be to use a d12 instead of a d6...
1 - - - - -Right Leg
2 - - - - -Left Leg
3-8 - - - Torso
9 - - - - -Right Arm
10 - - - -Left Arm
11-12 - Head
Aiming "over" partial cover would hit on a 9-12
Aiming "around" partial cover would hit on a 1, 9, 11-12 or 2, 10, 11-12 depending on which side the target was leaning around.