So after looking through things a bit more thoroughly (including the 1st Ed Combat & Tactics, 2nd, and 3rd ed rules), here's what I've settled on:
Called Shot (Disarm, Groin, Sap, Staple, Sunder, or Trip):
Attack with disadvantage (AC varies by DC) Target saves (type varies) or is dazed, prone, incapacitated, or restrained until start of your next turn.
Sunder causes damage to item not creature.
Cannot cause extra damage or bypass AC.
The AC to hit is determined in the same manner as DC for other actions. In most cases I'd probably rule an AC of 15 or 20.
The specific condition caused depends on the attack. For example, a groin attack would cause the target to be incapacitated, where a sap would cause the target to be dazed (Oh, I added Dazed - attacks, saves and ability checks with disadvantage). A target stapled to a wall by arrows would be restrained.
I have other conditions (knockout and stunned) as potential conditions caused by a critical hit, but not for a called shot. They are too severe to allow for just an attack with a penalty to hit.
Randy