[PseudoFenton]

Not all parries are violent collisions between the two weapons. Deflecting the force of the blow is generally the point of a parry - stopping it short by interposing your weapon can do that, but is harder and would (as you suggest) damage the weapon in question.

There are plenty of rules in GURPS already to handle weapon damage (thanks to your ability to just outright aim for an enemies weapon), so you can just use those. Although I'd say that a successful parry with a margin of 1 or 2 would result in damage, whilst any larger margin represents "proper" deflection of force and results in no damage.

It still lets you notch, dent and chip bits of each others weapons even when aiming to hit the wielder, but doesn't degrade weapons too quickly. That, and/or apply a large penalty to damage rolls (perhaps -2/die), otherwise Parry becomes a very powerful defence option, and you'll find everyone just ignores hit locations and sinks all their skill into deceptive to prevent their weapon getting trashed (or just adopts a ranged only combat style).

EDIT: That'll teach me for reading the post properly - sorry, you suggested declaring a parry first and only damaging a weapon when their attack fails but your parry doesn't.

This makes a lot less sense, parries don't generally try to hit weapons that are already going to miss, otherwise you're playing tennis with the enemies weapon! Aggressive Parrying and Stop Hits sound more like what you're attempting here, which would allow you to try and hit the enemies weapon, Martial Arts 65 and 108 has more details.