Assigning Navmesh Layer to Multiple Objecs At Once

Hello,
How do I assign nav mesh layers to multiple objects? In the Navigation pane, it is possible to choose a navmesh layer only if the selected object has a Mesh Renderer component. How do I assign the layer to a group of objects parented under a null object? I can try to write an Editor script to iterate over selection, but it is so basic a functionality that I am certain I am missing something here.

Best!

 

Nobody knows?