If you can find a plastic tube the right diameter to slide over your brake cable and through the frame tube, that will make it easier.
Even if you can't, fishing the cable through probably isn't as hard to do as you're thinking. The trick is to position your bike so the cable exit is on the bottom of the tube. That way. gravity will position the cable where you can snag it and pull it out with something.
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My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.