Mmmm! That's not an easy problem to resolve.
I notice that the frame was built by CYFAC which, at the time that your frame was built , means that the builder would be Francis Quiilon, the founder/owner of the company, based near Tours in central France..Some of these frames were also bulit by Steve ? and Geoff? two ex=Carlton frame-builders who set up Columbia Cycles in part of the former Carlton factory in Worksop.
However whoever built it should have brazed up a brake cable tube using two steel ends, joined together by a length of cupro-nickel tube - a bit like high quality automotive hydraulic pipe..to avoid internal rusting.
The question remains of how to remove the rusted up outer brake cable. My suggestion would be to take something like a long stainless steel spoke. or even a length of 1/16" brass brrazing rod and use this as a prod..enetering from both lends of the internal tube. The spoke will have its cranked end that will be very useful for scraping around inside, while the brass rod could have a small hook bent up on the end with a pair of pliers to serve the same purpose. As the internal tube is likelt to be no more than 20" in length you should be able to prod its whole length by working from both ends.
it was very much in vogue in the 80s to use internal brake cable..and gear cables too on time-trial bikes. In my experience , mainly from building such devices into frames, the actual tube internal to the top-tube only has a very shallow banana-type curve along its length which should permit the entry of the spoke.brass rod.
It's worth a try...Carry On Prodding.
If you are looking for an effective rust preventative for treating the inside of your frame's tubes, why not use Waxoyl? It's readily available. However in the event of using it then, at some later date having the frame resprayed..remember to tell the sprayer about the internal treatment.
