I searched and didn't find anything specifically about this question.

I have 2000 2500HD with a 6.0L and while staring at the calibrations in HP Tuners I noticed GM locks the converter in 2nd gear at nearly WOT in Tow/Haul mode. It's set such that a 2-1 downshift can't happen so you are to the firewall on the pedal but well above the 2-1 speed and TPS.

I was pondering why they might do that since it seems like going with the lesser of two evils (trans temps vs converter stress.) But if the converter can handle it, why not lock it up at say 50-70% TPS?

Can the converter handle being locked at lower throttle positions under a towing load (say under 8000lbs)? Many places I go I have to drop to 2nd (or even 1st if the road is twisty enough) to stay near the torque peak of around 4000 RPM.

My trans cooler does a dang good job of holding the trans temps under 230 in extreme situations and I usually never see much over 215 or so (4x stock in size). I was thinking that being able to lock the converter in second at less that WOT would help reduce the temps even more. If I need more that 70% TPS I just drop to 1st and go up at 35 MPH. I am not proposing converter lock up in 1st.

Just wondering....

It's set such that a 2-1 downshift can't happen so you are to the firewall on the pedal but well above the 2-1 speed and TPS
We know that downshifting will not happen while you are in a mechanical lockup. Therefore when you are closer to wide open throttle you are closer to matching engine rpm to transmission input rpm, mechanical lockup occured and you cannot downshift because the computer will not allow. Why? because of the torque of the engine braking.
You matched engine speed and transmission input speed and a lockup occured.

But if the converter can handle it, why not lock it up at say 50-70% TPS?
The converter lockup is not designed to handle loading from the engine per se. " ie you putting your foot into it from zero to sixty. What the automotive industry came out with was a economical answer for lock up to occur at highway speeds. This being the primary purpose and an answer to dissatisfaction of poorer performance of manual transmissions. Mileage and highway power loss, especially over long distance.

To reanswer your question again from another angle:
But if the converter can handle it, why not lock it up at say 50-70% TPS?
To an eighty year old lady the lock up torque converter would last for years and GM would never have any customer dissatisfaction.
To the teenager who wants to burn tires big problems would arise because the torque converter would last a very short time. If by chance the engine and the transmission speeds met at relative close speeds they would be too short. The enertia plus the power from the engine from one direction and resistance from the transmission from the other direction would kill the torque converter clutch in very short order.

So the answer is no, it can't handle it.

One point, I believe the converter will not unlock on a light or manual 4-3 downshift. Doesn't help me though!

depending on the type of truck you have.
It unlocks depending on the gearing.
When you step on the brake.
When you release the fuel pedal.

depending on the type of truck you have.
It unlocks depending on the gearing.
When you step on the brake.
When you release the fuel pedal.

yep