Lets assume that you already have a frame with internal cable routing holes because, of course, we would not condone drilling your existing frame. In that case, you would run a derailleur wire into the front hole towards the back and then after removing the seatpost and sticking a fabricated long nose pliers into the TT you would steer the wire out the back hole. Then you would clip the stop end of the derailleur wire off and run your brake housing through each hole guided by the derailleur wire.