In our group we have included optional rules for things like this. However, to sustain this kind of critical damage the character usually needs to be on 0 hp or receive a critical hit. The rationale behind this is that hit points represents a characters ability to defend and not so much his ability to "take hits". So, as long as there are hitpoints to spare the character have effectively avoided any serious hits, and turned potentially fatal and crippling hits into glancing blows or misses, but at the expense of fatigue or accumulating bruises. Once the hps are down to 0 (or otherwise incapacitated), you can in effect do whatever you like with the character and break his jaw or whatever. If the blow is a critical hit, the blow is assumed to land and do proper damage, but not necessarily enough to incapacitate him.
We use a list of random events including things like scars and broken arms or lost eye all the way to instant death. If the character doing the damage cant align the blow properly but does the damage in a fight or something, the damage done is random. If the character can align a blow reasonably well (head blow to a tied down character for instance, the recipient need to be incapacitated) he get to choose.
This sort of thing really brings out the usefulness of spells like Heal or Regenerate since they can be used to repair severed limbs.
If you want to include the random damage due to crits or reduction to 0hp the severity of the list sets a different mood to the game since getting hit can be much more dangerous.