Well, after a bit of testing, I'm tentatively using a minor add-on to the current rules. I'm adding in "fatigue," which adds up approximately like subdual damage, although for now i'm shaving down how much fatigue comes from an unarmed blow (like, maybe only if a hit is critical or exceeds the MDT, and then maybe not all the damage rolled).
When accumulated fatigue exceeds the threshold of Con, Current HP, or Max HP, the character moves through Staggered (only partial actions), Fatigued, and Exhausted. When Current HP equals Max HP, it counts as only a single step. If you lose HP so they're not the same anymore, well, that's another step for you.
I think this could get a decent compromise. Combatants will inevitably wear each other out, but won't necessarily score a knockout. By the time you're exhausted (-6 Str and Dex), there's not much point in fighting any more, since the average guy has no capacity to do anything by that point. Heroic types can struggle on, but even most PCs will give up the fight by that point, I suspect.
This also means that if you realize your opponent is at max hp, and therefore can't get worse than fatigued, you can take the -4 to deal a single lethal blow, which will propell him to exhaustion. No need to beat someone into a life-threatening pulp just because you can't KO 'em, but it still makes the Brawl tree worthwhile, since you can both wear someone down faster and have better odds of a knockout by massive, nonlethal damage. [I'm also trying to think of ways for exhaustion to affect how long a character is unconscious, but I won't worry about it for now.]
BTW, I'm adding this not to make things 'more realistic,' but because I think it will encourage my players to use non-lethal force in common situations. But I am hoping it would also serve as a middleground in all these "That's not the way real fights go" arguments. Feedback on that count is certainly welcome.
Once I can herd my players together again, I'll be sure to test this out further and report what I find.