AC For Body Parts (my players stay out)

I'm getting ready to run a new fantasy campaign using 3.5 with so many house rules that it would, frankly, barely be recognisable as such. I'm bringing back some elements from earlier versions, such as facing, and I'm introducing some extra elements. One of those is AC for specific body parts, and magic items that affect only those parts.

I currently have a plan on how this will work, but I'm not sure if I entirely like it or not. So my questions are: Can anyone else improvise a system, or has anyone else seen a D20 compatible system that does this already, and either way, how does it work?

Why? Do you want piece-by-piece armor? Do you want called shots? Do you want random (or partially random) hit location? There's lots of things I could suggest, but I'd like to have a better idea of what you goals are.

I think Torn Asunder by Bastion Press has a d20 called shot system and crit charts for different body parts that works fairly well as a D&D add on system.

IIRC (I read it years ago and don't use it for that system) the called shots have varying penalties for different body parts but only come into effect for critical hits. Critical hits can do more crippling stuff to various body parts if your confirm roll is 5, 10, or 15 more than you need to confirm the crit.