It's lovely when a simple question fires up the brain and starts making all sorts of connections. This was something that appeared in the charming Mr Kovalic's Twitter. It will be unsurprising that my path to HEMA included gaming and wondering about the realities behind the story.
The question that intrigued me was making called shots in melee. There are a lot of good ideas, but nothing that rang true to my experience of sword fighting. Let's ignore the fact that an offhand dagger should add quite a bonus to defence or that a buckler is also a bludgeoning weapon. Strength isn't the defining trait for a fencer. A fairly weak person has a lot of tricks than can counter brute force.
I haven't done P&P Gaming for ages so my knowledge of combat stats is THAC0 and memory as old as the name. Which means numbers are not my domain.
If dice rolling gets people engaged then willpower (holding when someone is coming at you) Charisma (taunting) or perception (spotting openings) rolls could be an option. This could allow effective fighters that are more than walls of meat. People fight in line with their personality.
The difference between called shots and non called is not just missing the shield. A volte into the ear or thrust to the eye are called. Leaving an engagement with bits intact and a hole somewhere in your opponent is the no frills version.
The idea works around the patient agent. Creates an opening through distance and timing then explodes into it. Initiative becomes important, if you have the wherewithal to attack first, you can wait and attack into a cut. (Countercut or hand snipe). also framing guards in certain ways makes it more likely your next action succeds. Basically the initiative loss is you setting up for the strike.
Give up initiative for the called shot. Each combat round inactive to add a bonus to initiative on the next round. Bonuses to hit based on mental skills, if gm pemits (so watching the opponent for tells would be a perception based bonus. Waiting for the last possible moment- will)
You may interrupt a fighter with lower initiative to make your called shot against them.
Effects vary against target, there's probably cripple tables or something to emulate this.
Some targets are easy than others hand sniping (assuming simple hilts) should give a bonus. Leg sniping is much harder.
A pommel to theface could be called, with lower damage (4 teeth worth of hit points) but would allow a free action\critical\backstab if successful enough.