Strictly speaking, tmux is out of scope of the question, but I hope some people would be interested.

I found myself needing to use deeply nested tmux sessions a few times and navigating between nestings is a huge pain under a vanilla setup. Using some configuration trickery, I was able to hack in keys to "focus" between nesting levels. Here's a repo I have it sitting under:

https://github.com/xelxebar/tmux-addons/tree/master/nesting

The code should be relatively drop-in and the README not terrible.

I'd love to hear thoughts.