Those holes in the picture were drilled then a braze on angled cowl fitted on top. I've had bmx frames that I beat to $hit with these tucked away cables and they were fine.

In all honesty, from my knowledge of metal working and frame building I would almost think that the holes are punched with very specific tooling, not drilled and the angled cowl is the reinforcer. However, I've not seen the process in person. Secondly, done right the frame builders route automotive steel brake line internally and braze it in a nice ovalised entry/exit eliminating the need for fishing line chasing etc.

My recommendation is skip it. Get some nice chrome cable stays or some clear 3m tape and fasten accordingly under the top tube. If you are dead set on drilling the frame then go for it, drilling in a place that you have seen other bikes with factory holes. Just paint inside the holes if you can and keep it out of the weather.

Lastly, you will not survive on the internet long if a thread such as this tips you over. I'm a sensitive cranky old man and I have a fairly thick coating of wax on my back. htfu. Recognise humour and real advice when you see it and smile more.