How do you repair or lengthen a B&M Luxos Control Cable?
I finally got this done. The procedure is as discussed in the chat for this question.

Open the light by unscrewing the two torx screws on top. (Be careful not to lose the rubber washers, they're transparent!
Take a photo of how things are arranged and connected inside. Putting the thing back together is a bit like playing tetris blindfolded.
Disconnect the cache batteries. They carry current, possibly up the switch cable, so you don't want to short something and fry the control circuitry while working on the cable. This light is fairly sophisticated, monitoring changes in voltage, ambient light levels, etc. I suspect it's easy to kill that circuitry with carelessness.
Cut the wire and insert an extra length of 4-conductor control cable, braided. I found some on ebay very cheap, 4 wires, AWG24.
Solder and heat-shrink wrap each individual connection, then heat-shrink wrap the whole splice area. (This means soldering 8 connections total.)

When you put it back together, the light will flash when you plug the battery back in. Don't get spooked. As soon as you connect the battery, you can test the remote (and onboard) switches to see if everything works.
Mine did. :)