I am about to flop 'n chop SR Modolo bend drop bars into bullhorns. I also have a DiaComp reverse brake lever (TT lever) which has internal routing. My dilema is...where do I drill the exit hole for the cable housing? On the back or the bottom of the reinforced section of the bar? As close to the stem as I can or farther out?

Thanks for your help.

I really wouldn't drill a hole, especially near the stem. It could develop a tear and crack.

Handlebars and stems have the potential for catastrophic failure, I wouldn't tempt fate by drilling holes. Buy a pre-drilled set, or don't use internal routing.