To oxygenate cool water, you need a way to oxygenate cold water without mixing the waters above. So it won't be standard aeration. I am reminded of how some indoor systems supersaturate water using high pressure injection of air or oxygen. If you draft cold water, saturate it, and then discharge at or below the thermocline ... Maybe?

As for the minimum temp below the thermocline, this would be the ground water temp. This would be the lower limit and could be low enough in a deep pond to sustain trout provided it is near saturation of oxygen.

It isn't what we don't know that gives us trouble, it's what we know that ain't so - Will Rogers