Is the R derailer cable external or internal routed ?. If external I be would be drilling a new hole near the rear of the chainstay IF you can route a cable into the chainstay from the b-bracket shell. I would be pulling the crank and b-bracket to ascertain that. Then another hole maybe on the back of the seat stay near the top of the F derailer for that cable. I had to do this in my aluminum hard tail to install Di2. Was not difficult. I had to enlarge the hole at the R derailer, my mechanical shift cable was internally routed so I enlarged the hole to run the E-Tube cable, the connector is larger than derailer housing. If there is no routing from the inside of the b-bracket shell thru the chainstay, I would be drilling a hole at the very bottom of the seat tube, then route the R derailer E-tube cable out of that and tie wrapped to the inside of the chainstay. I protective wrapped the chainstay with rubber insulating tape that electricians use