This topic comes up every once in a while.
DON'T DO IT!
I have personally seen bars break because of this.
The kicker is my buddy who had drilled bars and had them break asked me to drill his replacement bars. Yup, he really did and was serious. Showed up at my house, bars in hand and really pressed me to drill them. See, he is not one to use tools or do mechanical stuff so he was looking for someone to do the job. Wanted holes drilled right near where the stem clamps. Jeezo
I told him nope, it is not safe. They will break. I was blown away that he did not learn or understand that this was a really bad thing to do. I think he kept asking around until he found someone to drill the replacement
Maybe this was the thing to do in his cycling club. He had shared a story years later about a member decending Mt. Diablo that had her bars break. I never thought to ask if her bars were drilled.