the reason for the full length tube is to stop water intrusion into the tube. But water is going to get in there with the housing, so that's a good reason to use stainless.

Plenty of people build with internal routing, and a lot of riders want it. Granted, it's mostly for looks, but there is nothing wrong with that. Some builders put all the cables inside the tubes, which I always thought was risky. But just running one brake cable inside the top tube is very common, and unlikely to fail if you put a full length tube for housing in there. Otherwise, history says the chance of failure is fairly high if water can get in there. The top tube takes a reasonable amount of stress, so it's a concern.

If it's just for fun, then experimenting is fine. If it's for someone else, I think a conservative, mainstream approach is best.