The “all internal” routing you suggest seems cool but I am not sure you would be able to turn the steerer (you know, in order to steer the bike!) without essentially having it work as a scissor and cutting the wire off. On the other hand, I don’t see why you think the wire in the down tube would be particularly fragile.
This system was a standard feature on many touring bikes of the 80s and it still is a standard add on for most of today’s custom builders here in the US. I have a 1984 Centurion Pro Tour with the 36-years old dynamo wire in the down tube, pre-installed at the factory, still going strong. Maybe I’m missing something, but surely the wire feels safer in the tube than out!
I am not suggesting you route through the steerer to to the inside of the head tube. I was waving you off in case that's what you were thinking. I think we are both in agreement such an arrangement would be a train-wreck.
The hole in the downtube is a stress riser in a high stress location. Especially with the cheap single margin drill bits that seems to be the most common available at the local hardware store. A proper double margin drill bit &/or reamer & the proper cogsdill deburring tool
would be the proper tools for the job, along with a drill guide to ensure a straight round hole. Expensive city.
All that aside, the wire will flex at the downtube enterance hole with each & every correction in balance as you ride the bike. The wire will eventually break from all the flexing & that'll be all she wrote.
You do what you want. Drill the lug if you insist on making holes, (I wouldn't, nor do I recommend it) but as a machinist, I would not do so in the downtube.
I would count on replacing the broken flex prone wire regularly. Sure. The part in the tube will be fine. Nice & durable & protected. But it'll get 6 inches shorter with every fix 'cause the flex at the hole is the problem.