Since the rule book does not specify where grappling damage is applied, it is always applied to the torso. However, as opposed to normal Unarmed Attacks, Armour does not count double against Grappling.
The rule book states that wearing any mail type armour at all (except head armour) does decrease Agility by 10%. This is overruled by a house rule stating: If you have armor on the torso, that decreases agility by 2%. Armor on the legs or the armos decreases agility by 4% each. Thus, full mail armor still decreases agility by 10%.