I now realise, having looked up the various import procedures for 10.3, that it is possible to import folders and individual files from the Published Smart Folder by dragging from the standard Apple Finder to an event or directly to the timeline.
The two convenient ways to do this are (quoting from the Apple manual):
- Select a file, Command-click to select multiple files, or select a folder of files, then drag the file or files from the Finder to the event.
- Select a file, Command-click to select multiple files, then drag the file or files to a project in the timeline.
This method is not so convenient as in 10.2 because it adds an extra step.
And, just in case you are wondering, the latest update (10.3.2) which I installed this morning still does not restore the functionality in this respect of 10.2 although files of audio folders can now be added to the photos and audio sidebar.