I've been a Smugmug user for a couple of years and agree with your post. I think it could be accomplished by password protecting an image (single or batch) or by nesting galleries, which another user mentioned. Nesting galleries could not only accomplish this task, but possibly allow users to specify that a photo lives in two or more galleries. Unless I'm mistaken, the only way to show the same photo in multiple galleries right now is to upload it to each individual gallery. It's a bit off topic, but if someone knows of another way, please let me know.
http://brandonllewis.smugmug.com