I run my cooler independent of the radiator. I tow an 18ft toy hauler and when its loaded the weight hovers around the R50 max recommend capacity of 5k lbs so I wanted dedicated HD cooling. I don't currently have a temp gauge but I definitely want one. I monitor everything through my OBD2 real-time when towing but it doesn't have any trans readouts.
The only downside of bypassing the radiator for me has been that when temps are low, less than 32°F, for first few miles of driving after a cold start the transmission wont shift into OD while it gets up to operating temps. This has barely been noticeable since I cant reach any significant speed (legally) on the country roads around my house before getting on a highway. Other than the cold start scenario there have been no adverse effects of this setup.
As for the temperature controlled valve system- I personally would not ever trust a system like that because if it fails and I'm towing or miles away from any civilization and services on a 4X4 trail the transmission failing could potentially put my family and I in serious trouble.
Now, on the other hand, a manually actuated valve @onespiritbrain mentioned is appealing and pretty easy to do. Same design and functionality of older vehicles with a manual choke.