The goal has always been to have as much internal routing of the cables as possible, but to hopefully not cause catastrophe by weakinging the aluminum. Besides the cleanliness of the look, this being a commuting bike means that the more internal connections there are, the less there is able to steal off the bike. The big thing is the battery, but external junctions are at risk too. I have two paths in mind for the routing. The first, is running the cable from the display THROUGH the handlebars, throught the stem and then down the fork meeting the internal junction in the downtube. This would be the cleanest way. Problems would arise with drilling holes in the aluminum bullhorn bars. There is definitely conflicting reports on both sides of the argument, but really why risk changing the structural properties of the handlebar and causing them to fail. I could just buy predrilled handlebars, but would still have to have a hole in the center of the bar opening them up to the stem. The second option, which may be a little safer, is to drill a hole in the down tube and route the cable there, bypassing any holes in the handlebars. SO far, I am leaning towards option ONE and made a few routing holes in the fork. If I stick with option one I will prbably get steel or predrilled bars for some piece of mind.
As you can see below, I used a 13/64ths bit to make the first hole which lets the di2 cable slide through perfectly. Then i opened up a 45 degree angle with a 7/32 to ease the angle the cable needs to pass through.