sdkhooks has a hitbox parameter on some functions you could use. I think the item that blocks headshots in war3source actually uses this to block damage if hitbox == head, maybe that can help you.

for future reference on hitboxs

Hitbox
0 = head
1 = pelvis
2 & 3 & 4 & 5 = Spine (backstab related)
Left Arm
6 = Upper arm
7 = Lower Arm
8 = Hand
Right Arm
9 = Upper
10 = Lower
11 = Hand

Bottom Half Left
12 = Hip
13 = Knee
14 = Foot

Bottom Half Right
15 = Hip
16 = Knee
17 = Foot

Hitbox Group
0 = Pelvis
1 = Head
2 & 3 = Spine
4 & 5 = Arms & Hands
6 & 7 = Legs & Feet