There's no such thing as a guest pass or a private link to sets. That's the reason why I switch to Picasa Web Albums for scenarios like yours.
But: As there is no access control on the file level you could hand out a list of direct photo links i.e. is to JPG files themselves.
http://static.zooomr.com/images/4669207_1708be199f.jpg for example links to a private photo of mine. If your list is long this might be none too practical though. And those links only expire when you delete the photos shared in this way.