On the flip side of your argument, you mention Krav Maga, Pankration, and Mau Thai, all of which teach various methods of stunning an opponent through massive blunt force trauma to the head or chest - the very essence of the Stunning Fist feat.
Ahh, here we come to the heart of our disagreement then.
I don't agree that 'massive blunt force trauma' to the head is the very essence of the Stunning Fist feat. It may be the very essense of how you really stun someone, but the Feat itself has a different genesis. There is two separate peices of evidence. The first is internal to the feat itself. The second requires knowing the history of the idea.
To understand the internal evidence, you have to compare the implementation of the feat to a feat inspired by the concept of 'massive blunt force trauma'. What would a 'stunning' feat look like with a Western Martial Arts spin on it. I would suggest something like the following:
Stunning Blow [General, Fighter]
You been trained to focus your blows to keep you foe senseless and reeling.
Prerequisite: Str 13, Power Attack, BAB +1
Benefit: When using a unarmed strike or bludgeoning weapon, if you successfully make a critical hit while using Power Attack, then your target must also make a Fortitude save with DC equal to 5 + damage dealt or be stunned for one round.
Ok, so that's just off the top of my head, but you can see from the implemetation that the underlying idea is, "You make a called shot at a sensitive target, and, if you strike it forcefully enough, then the target may be stunned."
But look at the implementation of 'Stunning Fist'.
Stunning Fist [General]
Prerequisites: Dex 13, Wis 13, Improved Unarmed Strike, base attack bonus +8.
Benefit: You must declare that you are using this feat before you make your attack roll (thus, a failed attack roll ruins the attempt). Stunning Fist forces a foe damaged by your unarmed attack to make a Fortitude saving throw (DC 10 + ½ your character level + your Wis modifier), in addition to dealing damage normally. A defender who fails this saving throw is stunned for 1 round (until just before your next action). You may attempt a stunning attack once per day for every four levels you have attained (but see Special), and no more than once per round. Constructs, oozes, plants, undead, incorporeal creatures, and creatures immune to critical hits cannot be stunned.
Special: A monk may select Stunning Fist as a bonus feat at 1st level, even if she does not meet the prerequisites. A monk who selects this feat may attempt a stunning attack a number of times per day equal to her monk level, plus one more time per day for every four levels she has in classes other than monk.
Note that this is completely different. There is no mention at all of making a more difficult attack - a 'called shot' if you will - as in the alternate implementation. There is no mention at all of the attack needing to be particularly successful. There is no relationship between the ammount of damage done and the stunning effect. Instead, if we look at idea, we see that its something you can only do a limited number of times per day and then its expended like a 'charge'. Even if you fail, the 'charge' is still expended. Moreover, the ability isn't tied to damage, but rather to the character's 'level' or some intrinsic power in their being. And moreover, it can only be done with a bare hand and not say a lump of spiky steel. And further, Monks get a big bonus on the number of 'charges' that they can produce and on their access to the feat. So clearly, there is something inherently monkish about this power that masters of martial arts (eastern martial arts) would have an huge advantage in their understanding.
All of this points to the underlying concept not being massive blunt force trauma, but rather the utilization and transfer of chi.
And we can very much verify this by tracing this the origin of this particular rule back to 1e, and find that intend, the original 'stunning fist' was part of the portfolio of the Monk specificly and that the Monk had a wide range of supernatural abilities all related to chi (as is made explicitly clear by the Oriental Adventures rules).
