The fluid runs through the cooler all the time the engine is running. Thats where the temp sensor is located, and thats why the trans temp drops quickly once locking up.
I didn't miss your point. In your example above, when the TC locks, the TC has stopped putting heat into the ATF so the box will start to cool down regardless of flow through the cooler or not unless something else is keeping it warm. The ATF only needs cooling when the TC is unlocked.
If you do get stuck for an explanation Lorin you could try speaking to Rodney at Wholesale, you do have one of their valve bodies etc after all and I always found him very helpful when I was fault finding on mine.
That's one of the reasons I think no flow through the cooler when locked makes sense. Another reason is I suspect the cooler flow and lockup function are the same circuit to be able to fit all the functionality required up that little shaft the TC sits on but I am guessing / speculating on that one.you would have though it would be temperature controlled to stop it from running too cold!
I had a similar issue with my H61 at high speeds ( well relatively high speeds!), the ATF was getting hot due to the route of the pipes to and from the oil cooler. They both passed really close to the exhaust down pipe from the turbo. Lagged the ATF pipes and fitting an exhaust shield seemed to do the trick!
have you tried increasing the line pressure? the lock up clutch may be slipping with the extra load going from 60 to 70mph and just need a tad more grip. Just a theory, but 5 min job and free