Great minds, I am thinking of doing this on my next one. Only thing stopping me is the Di2 battery and relocating it out of the seatpost, maybe the steerer tube.

I will put a port at the top of the downtube either on the same side as the rear brake hose or the other side not sure yet. The cable will run internally down the down tube through the bottom bracket and up the seat tube. There is enough room to run the cable above the bottom bracket bearing assy with a little grinding on the seatube down tube junction if you are using a BSA BB shell, PF30 EVO86 have enough room.

You could run it externally to the bottom of the seat tube and internally from the bottom of the seat tube. There are plenty of different cable mounts you could braze on along the downtube to hold the cable in place.



They also make dropper posts that run the cable externally all the way to the top of the seat post, not a very clean look.