Hi Shaun ,
Sketchup 2018 supports filling the geometry cut by a section plane. I just glanced at the SketchUp API documentation, but I couldn't find a method to read these section fills in an easy way. We would have to perform the sectioning ourselves (needs some code and might hurt the performance for larger models), so I think it's unlikely to be implemented in the short run.
Right now there is a difference in how we handle section boxes in Revit and section planes in SketchUp. Revit cuts the geometry itself and adds the filling, so Enscape only knows about the visible part (this also means that only the visible part of the model casts shadows). SketchUp on the other hand does not modify the geometry for plugins, so we simply do not draw anything at the wrong side of the section plane (but Enscape knows the complete geometry and everything casts shadows).
There are a few people on both sides of the fence, who want to have it the other way. So It might be possible that we add cutting geometry ourselves (but not anytime soon), and in that run we might also be able to implement these section fills.