I strongly recommend that you DO NOT impose such rules.
If, however, you do, make every hit a locational one, else Hong's Paradox(TM), as listed above, will be manifested.
You should use a d20. After all its a d20 System.
In the case of an untargeted strike:
Random Hit Location
1-4 Left Leg
5-8 Right Leg
9-10 Left Arm
11-12 Right Arm
13-15 Torso (Lower)
16-18 Torso (Upper)
19-20 Neck/head.
A specifically targeted strike - i.e. a called shot should expand from existing mechanics. So:
1. All called shots immediately incur a -4 penalty to hit (logic: strikes to subdue, which involve a departure from the normal ebb and flow of combat, already incur a -4 penalty. It seems reasonable to apply the same penalty to called shots.) You could invent a feat called "Improved Called Shot" to offset this penalty.
(You also might want to consider whether Called Shots provoke an AoO like trips or disarms - obviously, mitigating feats could be developed.)
2. This -4 penalty is further modified by the size of the target: -1 for leg or torso (Small); -2 for arm or head (Tiny).
2a). In the same vein, you can add a penalty of -4 to hit a hand (Diminutive) or -8 to hit an eye or ear (Fine). Although, I'd restrict the option to hit these areas to small, one-handed piercing weapons (and maybe the rapier), or you'll get:
"Uuh, I'll hit him in his left eye with my greataxe."
Pages 67-68 of the DMG lists the effects of specific injuries to specific areas. Go with these.
Personally, I dislike the extra layer of mechanics. I tried it for a while, and I soon got bored of it.