Handling called shots

[Preposition_the_Noun]

During a game I ran this past weekend, there was a moment when a player wanted to target a specific part of an enemy(in this case, it was a magical hat that the player wanted to shoot off the enemy's head).  I couldn't find anything regarding called shots or even anything similar, such as disarming attacks.  To keep the combat flowing, I simply disallowed it at the time but promised to research it so we'll be ready next time the situation arises.  I still haven't found rules along these lines, unless I overlooked it.   I understand some GMs and even whole RPG systems don't like called shots, but since my players are the ones asking for them I don't mind house-ruling them in.  

My idea is to have the player attack with a  on the attack flip;  when dealing damage, reduce damage to 0 but the player gets the desired effect(disarming, etc.).  I'm also back-and-forth on whether I want the player to discard a card to use the option.    Does this sound fair, or does anyone else have any house rules to handle them?

[nomoredroids]

Skip the attack flip and just make it a challenging skill check (like a 13 or a 14). Doesn't cause damage, like you say. I wouldn't allow it to be a regular thing, but if there's a specific effect they're trying to offset and it's reasonable and makes for a cool moment then I'd always allow it. 

Whenever something outside the rules comes up, you should always just find a relevant skill, set a TN, and make them flip. The rules aren't supposed to be shackles, and in my mind telling a player they can't do something reasonable and within the theme/tone of the game is going to discourage creativity. 

[WDeseron]

If you look at the Bandit in the Core Book 2 ed., it has a trigger for causing a target to drop a held item.