Someone told me that bikes that use internal cabling have a tube inside the frame into which the cable goes. Otherwise (I am rather too familiar with aircraft control cables, for example) you would struggle to feed the cable through. You would need a long thin stiff rod to go in first - a bit like electricians use when running cables through e.g. the cavity of a cavity wall, to avoid having to chase out a groove inside the house.
In any case, if you were replacing just the stainless inner (which is likely to be less than half the job, because any stiction is likely to be due to degradation of the liner inside the sheath, not degradation of the steel cable) how would you pull that through the sleeve, if the sleeve itself is in multiple sections, whose ends may be inaccessible.
Does anyone do cable lubing? It is in principle possible, by injecting oil into the end of the cable, using a special adaptor.
