Hi,
thanks for the question. This does work in Forest Pack, but unfortunately not yet in RailClone. The reason is that Corona has to add support for the updated RailClone API. I'm afraid we cannot do this from our side.
We added support for non-geometric objects to the RC API for the new releases, but I don't think the Corona team have yet to add native support for this feature. As a workaround, we have added a function that adds lights in the scene in pre-render, but it is that, a workround, and it isn't compatible with lightmix sadly.
On the other hand, Forest was different because its API always supported scene nodes since its first version so no change was needed on the Corona side. RC instead, initially only worked with meshes... not scene nodes
In short, we need Corona to make some changes on their side to support native light instancing using RailCone, which should also bring with it compatibility with Lightmix.
Best,
Paul