I devised functional cable routing on the Serotta. It's not just outside the box, it wraps around it and sticks back in its left ear.
Does the box have ears? The walls supposedly do.
The internal route would not work. The housing crosses from one extension to the other and then curves around to enter the cable stop on the opposite side of the frame from the shifter. It's a variation on the standard cross-routing we're been doing for years. The cables cross back under the down tube to go to their requisite derailleurs.
Note that the cross attachment points are not symmetrical. This is not just sloppiness. It also allows the cable housings to flow as smoothly as possible past the front brake housing when the handlebars are turned. As smoothly as possible still isn't very smoothly, due to the inherent unforgivable inadequacy of the cable stop placement, but it's the best possible.
Sorry for the fuzziness in this picture. The complex surfaces and dark background really mess with auto-focus cameras.
Fortunately this rider does not want a water bottle hanging between the bar extensions. Even if she did, I could probably work around it.