I, too, used BritAudio for their Cardas wire + Cardas clips on my 12.5. They did an excellent and fast turn-around including braiding the wires for the entire length of the run within the arm as well.
As for doing it yourself, there is a device called a tonearm wire puller. I have never seen one, but apparently that is what is commonly used for interior tonearm rewire jobs. If there is dampening material inside the arm, the required skills to accomplish this would be considerable.