in-editor cloning of game objects

How do you use a script to clone objects not during run time, but to just set up a scene in the editor?

You may want to look into custom inspector views.

In all editor scripts (you’ll need to place these scripts under /Assets/Editor) you have access to these editor classes that can do almost anything in the editor.

So create a custom inspector, add a GUI button to it that will clone something you want and you’re done.