Cable routing around bottom bracket
You don't need to put anything in there. If you're really concerned get teflon coated cables. 

Those kinds of braze on cable guides were "standard" issue on well made lugged steel bikes until the early 80's or so. ( Cheap bikes came with bolt on versions of the same guides. ) They worked just fine even with old style cables that were much rougher than current cables. ( Current cables are pulled through a die to make them as round and smooth as possible and are also generally a much more rust resistant steel. )