I do not see a way to assign the current selection (or the entire contents) of a given layer to its background layer. Is this possible?
thisLayer.background.paths = [x.copy() for x in thisLayer.paths] # you need to copy the node, too.
And so on for all things you like to put in the background.
Or what I like to do:
thisLayer.setBackground_( thisLayer.copyDecomposedLayer() )
… to have reference layer of what the glyph originally looked like.