If it's a round or rounded tube I would not advice drilling into it. The rounded part is structural, and if you drill a hole there, the metal could start cracking or tearing. If it's a tube with a flat side then it's OK to drill a hole on the flat surface.
As others already pointed out, having a long cable housing is not great for shifting. So running the cable housing all the way from the shifter to the derailleur is not a good idea. But running the brake cable housing through the top tube is routinely done and not a bad idea.

I had a co-worker last millenium with a higher end, Miyata sport bike that he broke repeatedly at the brake cable exit just forward of the seattube. He was 200 pounds and rode it every workday for a 40 mile round trip. (Viet Nam vet with PTSD, issues and no license.) Miyata kept giving him new frames but seemed slow on the learning curve. (This may have been a case of exiting the cable forward of the tubing butt.)

If you run continuous housing, the effect on rear brake performance is the same as running that housing through TT braze-ons like was done for so many thousands of 1980s bikes. Well except the internal route requires sharper bends at entrance and exit. The TT full housing run is my preferred because it moderates the effectiveness of the rear brake relative to the front. (Less power back there, more control in hard stops. Fewer crashes.) Running bare cable completely unseen with bends at each end always seemed to me a very bad idea, especially for a brake. I'll run continuous housing secured with clamps on bikes with stops for bare brake cable runs.

Well I decided to go against everyone's advice and....just kidding. It stays as is. Glad the cables are on top of the tube instead of underneath, that is even more annoying and definitely good reason to route internally.

I like where you’re going with this, although I think we should establish a separate /sub for them in Mechanics (I propose naming it “Hold My Beer”)
That way, if they pull a Moisture or TimTak, there’s a layer of insulation from the general knowledge base and their well-meaning but wrongly informed zealotry.


*Actually, I think we handled this thread pretty well; after the opening salvo from the militant curmudgeonry, there was some decent conversation “I wouldn’t do it, but if I was going to; here’s how I would try it”

Because Bike Forums.

Too bad, you fail anyway. Being reasonable on BF is being unreasonable!