Internal cable routing blocked on '80s steel frame. What are my options?
I'm sad to say that I solved my issue by adding a clamp-on cable stop much like this one:

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If going down this route an optional extra is to poke the blockage with such force and frustration that your poking stick (coat hanger) breaks off and is stuck forever inside the chainstay. I elected to go down this route, but my understanding is that it's not compulsory.