[EricVanWyk]

I think you will encounter rule issues if you take this approach, as the Spike is called out specifically in several places as to how you can legally wire it.

I'm reasonably sure the spike has optoisolators on its input, but its been a few years since I looked too closely.

Personally, I'd just put a N-fet in line: 6V+, light, FET, ground. Attach the gate to a GPIO.