I missed that. You want internal routing? I mean, I get that it looks nice and all, but do you assemble your own stuff (or do you pay some poor sod to do it, in which case it's no problem)?
I assume, we're talking mechanically actuated rear mech (thick Bowden tube and all, is my point)? With that particular flat-piece half yoke thingy, there is no way to go 100 % internal. But why bother? Nobody will ever see those 20 cm of exposed cable behind the chainring / under the bb. I personally wouldn't even bother with reentry/exit on the chainstay, but I digress.
It could be done with a hollowed/tubish yoke piece (as Waltly suggested, for other reasons (and also advised against)), probably something they need to machine (that sound right now is a spinning $ wheel). You would probably lose a bit of chainring clearance (most of it except for the outermost position or so). Alternatively with flat bar, a split design (think: channel from bb shell to chainstay) might also just barely work, if you're a bit adventurous. Any of that would be it's own medium-level pita for very little gain.