A few thoughts:
- Stocking insects that give you the hatches you want might not lead to reproducing insects since they may not be suited for the tailwater and unlike trout which can grow in a stream for years even if they don't reproduce, insects don't live as long. If an insect wouldn't reproduce in a stream, how many would you need to stock to make a difference?
- If the browns have been able to establish I'd guess it is a fairly healthy stream and should support good insect populations. Insects can travel in ways trout can't. If the system is good and surrounding rivers and streams have the bugs they'll find their way and if it is a good fit for them should establish. I bet they are there though the bugs and their hatches might not match those on non-tailwaters. At least here in Colorado tailwaters are often tiny fly fisheries. Not sure if it is regional or a result of the artificial climate of the stream - warmer in the winter and colder in the summer than surrounding waters.
- A great way to feed a tailwater fishery is to feed the reservoir. A lot of food gets washed out of the reservoir and into the tailwater below fattening fish on food that wouldn't grow in the stream itself. Mysis shrimp
and the Frying Pan are a great example of this.
These are my non-scientific thoughts.