Stop. Stop. Stop.

I'll try to be gentle. Clearly you have not thought this through.
Take the time to look at a bike that actually has thru-top-tube cable routing. you will see that it's not a matter of simply drilling a couple holes in the top tube. The cable/housing enters and exits at a very shallow angle through a special piece (boss, fixture, braze-on, whatever). There is normally a "tube within a tube" here. The cable/housing isn't simply in the open frame tube. I currently have a steel framed bike with internal cable routing and this is how it is made. I used to own an aluminum framed bike with internal cable routing and it was much the same.
Doing as you suggest, well, you would have the cables entering/exiting at a 90 degree angle which would not only look like crap, it would put an extreme bend in the cable which would have a very adverse effect on your braking performance. Your inexpensive SS frame would be basically worthless at that point. I wouldn't even do this to a Wal-mart POS. Anyone looking at your DIY "project" would have to quickly turn away for laughing hysterically.