The issue with Safari is caused by the “block cookies and other website data” preference which is set to “From third parties and advertisers” by default.
Update: on Safari 8, this option is “Cookies and website data: Allow from websites I visit“.
The expected behaviour is only available if the user sets this option to “Never”, as seen in the following picture: https://i.stack.imgur.com/AP4ed.png
It’s unlikely that there is a loophole unfortunately – the intention of the setting is clear.