...I just did a bunch of internal cable routing to repaint and reassemble a Casati frame.
You hear a lot of stuff on the internet that has never worked for me in doing internal brake cable routing. The most effective way for me to do this (and what I did on my Casati) is to get a long piece of single stranded wire that is stiff enough to run from pushing on the rear, but easily bent so you can fish it from the other side.
Run your wire backwards from the direction you want the cable to eventually pull through. either measure before you insert it about where the exit hole is and mark it on the wire, or if it's heading toward a dead end anyway, use that as your guide. Twis up a short piece of the same wire, with a small hook on one end (needs to fit through the hole) and a loop on the other end you can hold it and turn it with,
Then insert your hook into the exit hole and fish out the wire at that end. Attach the cable end of the cable you want to pull to the end of your wire with a length of small diameter heatshrink. Use the wire to pull the cable. But honestly speaking, I'm with Andy on this. I've never yielded to the compulsion to run brake cables inside the bar. I only did it inside the top tune on this Casati because the fittings were already brazed into the frame, and the derailleur cables run internally too.