I thought about that, but it's a lot of extra wires to fit in. My Strael has an extra long wire to the battery and it's a bit of a fiddle to get the seat post back in. The bottle mounts block the tube on the inside, I doubt that a junction B would fit past them.
You only need to get the plugs through the holes.
Use extra long cables and route them all out the top of the seat tube, plug them in, then drop the junction B down.