I have spliced Di2 wires and it is not hard. 2 wires a black and red inside the outer insulation. The wires are small you will need to be careful not to cut too many strands when removing the red/black insulation. The outer insulation is pretty tough, used a new razor blade to remove.
You could consider drilling the frame by the FD, there is a thread on here where this is done to a Salsa Cutthroat.