Let me start by saying that I do know that the hit point system is abstract, but how would you deal with a lost limb or something of that nature? Surely it would incur ability score damage if you had...say a broken leg (Dex) or you took a rather hard blow to the head (INT maybe)...but how would one deal with severed limbs? I'm working on a project for my steampunk setting that deals with prosthetics and I am having trouble adjudicating when a character loses a limb or an eye or something of that nature. In addition, how does having an artifical limb affect your pool of hit points? Should it "remove" a certain amount from losing a limb, then the prosthetic would add them back, possibly even adding a couple more depending on the craftmanship of the limb. Should it have its own seperate pool of hitpoints (Hardness is a factor)? Or should I simply let the system continue functioning as is, putting the limb in as more of an esthetic choice. Although adding a piston to an arm would increase the character's strength by +2 or Dexterity by +2 if attached to a leg...An artificial eye could be built with aura sensors, allowing detect magic at will. An artifical heart could help filter poisons...just to name a few examples I am working with. Your thoughts, forum?
Skull and Shackles has some optional rules for that.
Game-wise, you aren't really specifying that you're attacking a specific body part, and it doesn't really care where you say you're hitting. It's just going to subtract hit points (unless you're using the optional Called Shots rules).
Your other questions are better suited for the homebrew forum, so you should go ask them there. 4 Winds Fantasy Gaming has a product that touches on the loss of limbs though. This one covers the loss of limbs, and this one has a bunch of prosthetics.
Rite Publishing Way of the Kappa features a racial paragon class for kappa, called the Bone-Breaker, as Japanese folklore noted kappa for their wrestling and bone-breaking ability - so it's built into a 20 level class.
However, the way broken bones are treated are as Dex damage or a -5 to movement for every 4 levels of the paragon class (to a minimum of 5 movement) - to define broken legs. Similarly broken arms are Str damage, Torso (broken ribs) is Con damage, and at least for this class a shattered skull is death, but that's it's 20th level ability.
I suppose you could substitute Int or Wis damage for a broken skull.