I would be concerned about causing stress risers at the drill holes. The holes would need to be chamfered on both sides to hopefully prevent this. Even then there's the concern (for me) of warpage caused by the holes. The holes would need to be drilled so as to have them equidistant from each other, then the drum should be rebalanced for best results. I would be concerned about dirt/sand entering the brakes through the holes in the event the vehicle was driven through puddles, etc.
All in all, I think your time and energy would be better spent elsewhere.