Two-tiered fisheries are an extra bonus for those who can maintain them. Your thinking is very sound, as always. Hypolimnetic systems have been in operation in Lake Ogallala, Nebraska for years to maintain trout fisheries during the hot Nebraska summer. One thing that a biologist told me is that just because you bring oxygen to the hypolimnion, you aren't necessarily going to have a great environment for trout. You'd also have to evaluate other parameters such as other gasses that are present that fish don't thrive in, or bacterial decompositional byproducts that might alter pH or general habitability for trout. You're definitely right about the cost of the systems. They can be pricey. I don't know of anyone who used a system in a private pond. Interesting experiment. I wish you would come up with an economical way for a pond owner to do this because I would follow your model.
Would you like me to find the name of a biologist who is assigned to the Lake Ogallala project? The information may also be available on line because the Feds are involved in the funding.
