I always thought D&D magic armor and shields tasteless, as negative armor classes bored me. Recently, I reread this Brian's rule and I had a kind of epiphany. So this is my version of this rule, in addition to my variant magic shields.
Shields shall be splintered :
Anytime you are about to take damage (after a successful enemy hit but but before the damage roll) and have a shield equipped, you may choose to sacrifice the shield in order to avoid incurring the wound. This way, 1d6 points of damage is avoided. Since all weapons deal 1d6 points of damage, that means damage is avoided. In the case of bonus damage (because of strong opponents, as ogres for example, or magic weapons), only these bonuses are actually inflicted.
An Ogre attacks a Warrior and manage to hit him. The character decides to discard his shield, and therefore suffers only 2 points of damage (the bonus damage).
In the case of magical shields, invoking this rule means that you subtract 1d6 + Xd6 damage from the blow, X is the number of “pluses” from its enchantment. Thus, a +1 shield would subtract 2d6, a +2 shield subtract 3d6, etc. A magical shield can undergo a blow* per day before being damaged. In this case, the shield loses one “plus” from its enchantment. Thus, a +1 shield would become a normal shield, a +2 shield becomes a +1 shield, etc.
*or X blows, but it seems too powerful.
Magic shield "pluses" does not apply to armor class.