Right, the layer colors are not imported.

So there is no standard way to implement a layer structure to FBX?

But since nobody asked you for "That's probably not going to be a good match for what MoI calls "styles", which would more naturally map to materials" it seems to be not so missed. But if a parent/child object hierarchy is the way to Rhino, than why no use it? ;)

I think, if you could implement a way to get layer transported to Rhino, than you and the user can profit only. Meshing user will be happy, you will have some "meshing only user". MoI layer color are quite limited to use for rendering materials, new materials are need to be setup at the final render package most. From my experience my clients are using layer names for transport material informations like "shiny white plastic", "soft white plastic" and "white rubber" - how will this colors be setup to the MoI layer colors. Layer colors wouldn't be helped to organize materials. If not material names are transported, than my clients organize objects, for example "walls, interior version 1, interior v2, plants, trees, ...". During rendering I turn on/off the layer for rendering different version of a scene. The layer color helps to see, which design version is turned on, but it's not so important.

If you would find a new way to get the layer structure to FBX that doesn't work for Rhino, than it is not useful. Please try to find a bridge to Rhino, if you like your Rhino user and want to help them.