If you check out the hypolimnetic aeration discussions, you can start to see a way to oxygenate the cold water and maintain the thermocline.
That is what would be required.
Keeping trout alive through summer is a difficult task. In the northeast at some elevations it was, and perhaps still is, a common practice to stock trout instead of other species. In many ponds fish would carry year round. However, even without any harvest the mortality of trout generally exceeded 80 percent by the end of the second winter. It has long been recommended to harvest all spring fish and restock in the fall.
The most challenging piece of keeping the water quality needed will be the fertility of the BOW. If the water is too fertile 70 degrees will not be a low enough temperature to keep the DO where it needs to be. Night time respiration will consume too much DO and trout will die ... even with aeration.
I used to live in Colorado at an elevation of 5000'. We had an irrigation lake nearby with a maximum depth of 40'. The thermocline was in the 12 to 15' depth. I actively fished for trout in the summer and generally caught them until about the 1st week of July. By then they had a very good size to them. Of course I kept all I caught.
I would just say this, if you want trout Fall stock them. Don't let summer survival (or lack thereof) stop you or diminish the enjoyment you derive from having them.
