Is there a way to append to an external blend file? I know that bpy.data.libraries.load() allows me to append data from an external file to my current blender instance, but is there a way to do the reverse? Take something (let's say the default cube) and append it to an external file using the Python?

My exact need is to take a node group from the current blend file, and append it to an external blend file.
In Blender 2.78, bpy.data.libraries.write() will be an option to use, which will allow any datablock from the blend file to be saved to an external blend file.