Okay ..the way I NOW see it, you're going to rig a full flow in the return line. It's going to be bypassed when cold with a thermostat.
No need, imo. ATF is very thin and you shouldn't have to worry about it.
One thing about most oil thermostats that I've encountered. They all appear to have some flow through them when full biased in the closed position. With something like water (I've attempted to use them for coolant) or AFT, this renders them mostly useless as flow blockers. They can be used effectively as a flow diverter if one was using an auxiliary cooler run in parallel with an in radiator cooler. There you would have the closed return circuit running to the in rad cooler and the normally closed/hot open ports running to the auxiliary. You can even do this in line up or down stream of the in rad cooler.