Drilled handlebars for aero cable routing used to be somewhat common, and some even came pre-drilled. Like the Mavic bullhorn bars Lemond used in the 89 TdF.

So I have a very similar set of Profile bullhorn "base bars" that I'd like to use without clip ons. Drilling them for internal cables would be the nicest way to do it because it keeps the aero cables aero without running the brake levers upside down.

Pros:
1. It was designed for clip ons, so the tops should be reasonably reinforced for clamping and also for a small oval hole.
2. I can drill a tidy angled hole without any sharp edges or corners.
3. I could epoxy a fiberglass gusset around the hole (but that might just hide a crack from inspection).

Cons:
1. Wasn't designed for drilling per se, and maybe those old bars were thicker or less brittle.
2. I like my teeth.

I would be too distracted by the slightest click or creak to enjoy riding the bike. But people used to drill bars when aero levers took off in the mid '80s, and they didn't break all that often.

I so like the housing (or two) running alongside the front of the bar at 4:00 so it/they lie inside my 2nd knuckle that I might not use pre-drilled holes. Never mind the risk. The cables run wonderfully cleanly with any lever housing location or angle and come off the bar in a wonderful smooth curve.

I have had a "sacred path" failure. (Front tire patch to hands.) The consequences were enough for this lifetime and a couple of future ones. It doesn't take a very high risk for me to say "I'll pass".

How do you route the cable that way on a bullhorn bar with the levers upright?

I set up my bullhorn bike using interrupter levers as primary levers. Works great as the sharp bend in the cable is eliminated.
I don’t mind having the cable on the underside of the bar under the tape.

If you decide to drill, swab the inside of the hole with epoxy to seal the cut edge. Use an artist brush or Qtip.

Is the idea to eliminate any "stress risers" in the cut edge? This isn't likely to help. It would be better to chamfer and polish the edges of the holes... not so easy on the inner aspect.

EDIT: Here's an example of chamfering a hole to minimize stress risers. The chamfering on the inside was done with an "inverted cone" bur in a Dremel tool.


These chamfered stress-breaker holes successfully stopped the progression of cracks for several thousands of miles.

Dan

several options:
They make these bar end brake levers just for that purpose. Some are meant to run the cable inside the bar and some outside on the underside of the bar

Or, it you need to run barcon shifters at the end, you can use interrupter levers:

Or, as you said earlier- just turn the lever around. This works better on bull bars that have a flat top and a distinct drop section (instead of sloped shoulders):

^ In this configuration the brakes worked extremely well. The aero brake cable routing goes really smoothly along the bends. Whereas if you turned the levers around and force the cables to double-back on itself, I am sure the braking would be way worse.
Yet another option , is to run non-aero brake levers. Shimano did make non-aero levers, which were compatible with SLR brakes:

Let's see the rest of that bike!!!!

Correct if metal. I was thinking CF for some reason.

I was just about to post "that seems like a good idea for carbon ..." but then I read down this far (-:

Sorry, I missed the bullhorns. And I don't. Never owned bullhorns and probably never will.