I've always wondered why the standard for derailleur cable routing under bottom brackets is to not use any cable housing. In the past, I have seen others slip the cable through a very thin housing material then into the bottom bracket cable guides when there are no plastic cable guides. This, I'm assuming, helps the cable slide freely and also protect the bottom bracket paint and shell.

Example:


Fantastic! I'm in. But where do you buy it from!? Everyone I ask seems to "have a little" in their shop but nobody knows where they purchased it from.