There is a very thin plastic tubing available that was used as a separate liner inside unlined brake cable housing. Shimano still sells it in 6' rolls for about $2. The part number is Y80098100 and your LBS either has it or can order it.
I use it to cover the open runs of brake cable on the top tube of my bike but you could use a short section to protect the bb shell and give your FD cable something slippery to run inside of.