It's worth understanding how smart previews work:
When you edit a smart preview, the only thing that changes is the catalog - same as if you edited the original. In both cases, the image file is unmodified (barring xmp considerations I mean).
So editing the smart preview is equivalent to editing the original - nothing is or needs to be "updated" or "synchronized".
OK, it's true that lib previews will generally be rebuilt when the original comes back online (and xmp may get saved..) but otherwise.. - smart previews are just a different (condensed) source of image data - what you are editing (changing) is the catalog, not the photo source files.
PS - Most people want to make sure the original is back online before exporting or printing, if max quality is desired, but even then: no updating or synchronization takes place, just a re-rendering for export, or print.. (same as if they were offline, except a different source of image data).
Rob