Maybe you could detect voltage on the bulb side by using an optoisolator. I took a quick look on Digikey and did not see one built for this, but i did not look anywhere else. You would need some electronics to bring the voltage down to the few volts usually required to drive the LED side.
Most of the discussion I have seen on MySensors is about how to power these nodes in a light switch box. Doing it safely can be tricky. Search the forum on "in wall transformer" or "in wall" and you will get lots of hits. Maybe one of them will be helpful.