At Cyclery North we made a couple of frames with internal rear brake cables using simple open ports (ringed with a reinforcing lip and plugged with poor fitting "rubber" boots). We ram the housing full length. I didn't like the lever pull feel (mushy) and there was casing slap over bumps. In the few months I saw the bike I did, after delivery, there was no known rust... yet. The few Panasonics PICS I sold a few years later at my shop also suffered casing slap. In fact the one that served as the shop sample didn't sell for a few years. We ended up stripping off the parts and sold it as a bare frame after the test ride feedback of "what's that noise?".

I deal with a lot of current carbon bikes with internal routing. Some are not too bad for recabling and in use cable friction. But others should get triple+ the flat rate for a cable replacement due to their designs. Why should a shift cable take 45 minutes to replace? Because the internal routing will make you faster Andy