@DaltonST:
Okay, got it. Thanks for taking the time to look, and for providing a reality check.
@Joanna:
Thanks for the idea of using Windows shortcuts (.lnk files).
To clarify: For each book, you can add a shortcut to a folder on your computer. Drag & drop the folder into the Book Details pane, which adds the shortcut as a "LNK" format, and then click on the "LNK" to open the folder.
I had originally discounted the idea of using Windows shortcuts because I thought that if I moved any of the linked folders, the shortcuts would all have to be recreated manually. But it turns out that Windows can repair shortcuts automatically, even if the target has been moved to a different drive.
(And if for any reason Windows can't automatically repair the shortcuts, this
Google search can probably help.)
calibre 2.57.1 [64bit] on Windows 10 Pro