The maneuver is not ment to actually remove the shield from it's arm but rather to pull the shield, including the arm away from the user. The fact it's strapped to the arm actually makes it worse for the shield user. If the user was using a neckstrap it gets even more annoying.I'd still use Disarm. It isn't limited to weapons.
I'd give the defender a bonus, similar to what a Locking Gauntlet gives, since the shield isn't simply held in the hand, but is instead both held in the hand and strapped to the arm.
So counting the -4 for the item not being a melee weapon, and the +10 for the Locking Gauntlet effect, I'd say shields can be disarmed at -6.
That sound about right to you?
You shouldn't be looking at your opponent's weapons anyway.That would mean the shield user could just readjust his grip and spend his move action to 'fix' the shield on his turn. Then stab you with his sword that you aren't paying enough attention to.
I dispute the claim that axes are 'well-suited' to grabbing shields, just because of their shape. They are meant to be swing in an arc, not pushed in/out.
I speak out of real-life combat experience in these situations, although not real combat. I know that it does not transition into D&D easily in many ways, but I was thinking of a way to implement such a maneuver. In real life you would also have to consider the possibility of the axe hitting the arm if it happens to be near the top edge of the shield. And a shield could be used to protect the person behind you. Things that are hard to implement in D&D.
In line with real life progress in the axe used as a weapon I am considering making a new weapon, called the bearded axe. It's a battleaxe but with a x2 critical modifier instead of x3 and with a +2 bonus on disarm attempts when pulling away a shield.
Although perhaps in D&D there isn't really a need for pulling away shields since all they do is give +2 AC.