Hiya.
What about allowing a character to substitute one of their weapon skills for "Shield"; without it, using a shield is an improvised weapon, 1d4, no shield bonus that turn/round. But with it, it does 1d6 damage and you keep it's defensive bonus.
This lets someone frantically use a shield if they have too. It also lets someone who trains in it's offensive use to use it as a "normal weapon"...meaning they could, say, hold a lantern in one hand and the shield in the other and not be 'unarmed'. Or they could use one arm to hold and prop-up their almost dead cleric get out of the battle...still having their shield for defense...or offense if they have too...
PS: I'd make it just "Shield", and let all shields be used this way in order to keep it simple.
^_^
Paul L. Ming