Dynamo Lights

Ooh, another option would be to run it via downtube and then out the Di2 FD cable hole and up the seattube to the light. It would then be hidden by the mudguard (if I had one).

DT Di2 entry > Dropout Di2 exit .. simples!
Rear light on drive side seat stay, out of the way from any saddle mounted bag(s).