I decided to try making the next few frames with internal cable guides for the rear brake. I decided to use some titanium tubing to guide the brake casing the entire length for ease of insertion etc. I chose titanium because it's light and has a similar coefficient of thermal expansion to carbon composite.
Here is the rather cobbled fixture I used to cut the 20deg entrance holes. That's a carbide burr to cut through the tubing cleanly (and not get dull). Once I have my little Garvin horizontal mill set up this will be much easier and less nerve-wracking.