My Corsa Extra had external TT cable routing.

The hardest ones to feed, in my experience, are the "tubeless," including my D'Arienzo and numerous older Kestrel Carbons.
The most difficult has always been my Kestrel K40 Airfoil, as not only is the TT brake cable internal, so are the shifter cables.
You basically shove them in until you hear them inside, insert a long hook into the dime-sized hole in the BB, and then fish out the cables.

For the TT routing on steel bikes and the Kestrels (which have a spring-loaded widget inside), I've used thick weed-whacker line.
It has a natural curve and some tensile strength.
The trick is to get it to the opening, and spy the little bastid in there, get it through.
After that, I tape it to a long "blank" cable, and then run housing over it.