In addition to the things mentioned in the KEEP, I have an example spek fork with scripting support. I plan to discuss it with spek people, so maybe it will land in the oficial repo at some point, but for a moment it may serve as an additional example. See: https://github.com/ligee/spek/tree/spekscript
Yes, it is mentioned in the KEEP that the current implementation is JVM-only, but future implementations for other platforms are planned. The timeline is not known at the moment though.