I actually already use J118 in my STTNG for the 12v supply for the Led strips. The ac from the GI is only used (after being rectified) to trigger the transistor.
But along with a friend of mine we came to the conclusion after studying the WPC power board diagrams that the proposed solution in post number 7 would never work as long as they share the same ground.
So instead we built a small circuit using an optocoupler to trigger a FET transistor (RIL540N). This way we canturn on the optocoupler with the rectified GI, and the FET transistor is then triggered sending the 12v from J118 to the led strips via the FET transistor.
Doing it this way keeps both sides completely separated electrically and works like a charm.
Hope the above makes sense My technical English isn't the best