Going back to the OP, I used the cable tie trick successfully on my after-respray rebuild for the handlebar-stem combi internal cables but was unable to use the same trick for the brake cable in the top tube. The Pinarello DUE has 6mm dia holes but in a recess such that the face, that the ferrule bears on, is in a plane at right angles to the tube axis (too many words - see photo)
Instead, I superglued a 5/16" steel ball bearing to a piece of string and pull it along the inside of the tube using a 12mm dia neo magnet on the outside. I had a piece of paper between the magnet and the top tube to avoid scratching the fresh paintwork. There must be some bumps or other irregularities inside the tube which made me lose contact with the ball bearing, but I got round this by trying different paths until I succeeded.
However, when I got to the other end the magnet did not exert enough pull to get over the ledge around the hole, so I used a 25mm long stack of 3mm dia neo magnets poked down the hole to fish the bearing over the last bit.