The following bug has some information in it that may be useful to you:
http://bugzilla.gnome.org/show_bug.cgi?id=372738
I really need to get my head back into it and follow up on it... The
bottom line is that newer versions of GDM can restrict shutdown
privileges. I believe my patch also restricts the logout dialog, but
it's not a good solution, as Brian points out in a comment.
Maybe you can even flesh out the bug a bit with your requirements, so it
will be taken into consideration when it's revisited, eventually.
--
Hans Petter