Hey, emr! I don't know if you've ever seen any of Tony Knight's videos on YouTube, but they are well worth watching. He had the idea of using a spring around the outside of his tether cordage to give it both stiffness in compression and flexibility at other times, like when you are pressing against a branch. Here is a link to his prototype video.
http://m.youtube.com/watch?v=Fso_x0SIFrw
He is Australian, so I personally have to listen closely, but he is really, really smart, and his craftsmanship is at a really high level, in my opinion. He made three or four more videos that show him climbing with his finished product, and it is beautiful. I really liked his commentary in the prototype video, but I suspect he may have suffered some abuse from online trolls over it, as I don't recall seeing any other yet myself in which he narrates anymore. He lets the video speak for itself.
Were I you, I would make Tony Knight's video channel a favorite place in your web browser. All of his videos are well worth watching. I vaguely recall him participating in a forum at some point, but I do not remember which one.
Here is a 2nd link to a video of his finished flex tether.
http://m.youtube.com/watch?v=Y7z0WBpCgFA
Sorry for the lack of embedding, I'm working off of my handheld computer at the moment, so it's a bit more difficult for me to manage. I just wanted to make sure emr was exposed to Tony Knight's ideas and videos.
Tim