View source for Noah's Mugging Rules

==Summary of Rules==

1. A helmeted character cannot be sapped. 

2. As with a backstab, the victim must be unaware of the thief's presence. 

3. The attacker must also have unobstructed access to the back of the victim's head (that is, a halfling might have difficulty sapping a giant, unless he stands on a ladder).

4. The attacker must be within one size class of his victim, ascending (that is, a T can try to sap an S, but not an M; but an M could sap a T.  Same goes for an L. An L could sap an H, but not a G. A very quiet and sneaky G could sap whoever he likes).

3. The negative modifier for the called shot to the head (-4) mitigates the thief's bonus for rear attack (+4) so that a sap attack is made using base THAC0. For humanoids without natural armor, the head is considered AC 10; no armor, dex or shield bonus applies (although certain magical bonuses might).

4. After a successful attack, the dice damage is multiplied by 10% and subtracted from the victim's system shock score. The victim then makes a system shock roll with the modified score. 

5. If the victim fails his system shock roll, he then makes a save vs. paralysis.  Failure of this save indicates a knockout for 3d4 rounds.

==Example==
While Eorl is picking his nose, Norwit saps him.  The wererat makes his attack roll and does 4 points of damage using Wirschen's petrified foot as a club.  Eorl has a CON of 15 and thus a system shock of 90%, so he rolls his check at (90%-4(10%))=50%.  He rolls a 43 and makes no additional saving throw. Eorl takes the dice damage (4 points) and whirls to face his opponent.

Harald, who is standing beside Norwit, turns Eorl around and casts forget on him.  Norwit then tries sapping Eorl again. He once more makes his attack roll, this time doing only 1 point of damage.  Eorl rolls his check at (90%-1(10%))= 80%.  He rolls a 97 and immediately makes a save versus paralysis. Eorl's save is 12, he rolls a 14.  Eorl takes a point of damage and angrily turns around.

Harald turns Eorl back around and casts forget on him again.  He takes Gur out of his backpack and puts on her crazy class changing hat.  After making his save versus demonic possession and replacing Gur in his pack, Harald chooses to be a thief. He then deals a mighty blow using a footman's mace treated with oil of impact, doing 17 points of damage. Eorl doesn't bother rolling a check since (90%-17(10%))=-80% and moves straight on to his save versus paralysis which he makes by rolling a 16. Unfortunately, the dice damage brings Eorl to -6 hit points, so he must make a standard roll versus CON to determine consciousness. Eorl rolls a 3 which is less than (15-6=9), so he remains standing, but starts to cry.

Harald is out of forget spells, but fortunately Zombres has one left.  He turns Eorl around and casts forget on him. Harald prepares to take another swing when suddenly Gur pops out of his backpack and clobbers Eorl with a tire iron, doing 3 points of damage.  Eorl now has a functional CON of 6, or a system shock of 50%, so he rolls his check at (50%-3(10%))= 20%. The dice show a 72, so Eorl moves right along to his save vs. paralysis, which he fails by rolling a 5. Eorl collapses for 3d4 rounds while Norwit, Harald, Gur, Zombres, Evelake and Arctopolis divvy up Eorl's gold and Hooba steals his shoes. They then leave him for dead without realizing that Eorl's periapt of wound closure prevents him from bleeding to death, and after 7 rounds he drags himself into a cave, there to plot sweet, sweet revenge...

==In conclusion==

These rules include everything I think needs to be included: strength and ability of the attacker as well as hardiness and experience of the victim.  I have no idea how these rules would work in practice, but the process, at least, makes sense to me.