I ran my past campaign with a similar system in place. Since it was using the One-Roll Engine, every hit location has its own points it can take. Being hit in one location reduces the stress it can take.
I just added an extra wound box to each location, and called it the "overkill" box.
If you only have your overkill box, the location is unusable, but can be healed. However, "unusable" head or body means knock out, and death if no medical help is given. Anyone can finish you off. If it was an extremity, it was still attached, but needed healing. The character was at a penalty due to not being able to use the hand, in pain and well-advised to consider whether this fight is winnable.
If you had lost the overkill box to lethal damage, the location is destroyed. It might be severed, ripped off, or whatever, doesn't matter. When that's your head, upper or lower body, you make a new character. When it was an extremity, the character was knocked out, and bleeding both internally and externally. Death follows again if no medical help is given. Anyone can finish you off.
Special characters, including all NPCs and antagonists of similar ability, didn't suffer from non-lethal damage overflow from a single attack (meaning a non-lethal attack didn't become accidentally lethal, since it didn't fit the genre). Anyone else was out of luck if taking a lot of non-lethal damage
If you can implement something similar in your favourite system, go ahead. I predict it would be easier if you use some variant of the Massive Damage
and probably replacing HPs with a Toughness save Save. Barring that, you can use the Wounds and vitality
option. Also, it's IMO preferable to use the Armour as damage reduction
option with those.
A look at the injury
rules might also be useful.
Either way, good luck, and have fun