One way might be to require additional skill rolls for some manoeuvers, such as having to make an appropriate Athletics skill roll as well as a weapon skill roll - either roll failing means you blew it. Perhaps extension or modification of the Precise Attack rules.
Sounds like a sound idea, express the maneuvers in terms of existing skills;
Be cool to ignore min skill levels - allow any one to try the action.
Cats Parry
Strike at the attacking limb, which has the added advantage of blocking the strike. if successful.
Make it combine a defensive and attacking action (two actions) and a penalty -40% (precise attack).
If under attack (taking to account -40%) the attack strikes the limb also and parries the blow.
Damage as normal.
Duck and Dive
Roll under lowest of half (Dodge or Athletics) to avoid all incoming projectile missiles, the subsequent round those firing at you must make adequate perception rolls to determine your location before they may once again fire.
Not very good I admit, but they kind of using existing mechnics with out adding extra traits. The kind of thing any one can use at any time.
But I do like the idea of having a list of optional combat actions,
helps player get in to the feel of things.
Not sure about automatic limb loss though - best remember if a player can use it so can an opponent.