Quoting deloptes <deloptes@yahoo.com>:
looks like gnome specific. did you check permissions? I think this was in policykit
Maybe my original post was not clear: Hibernating works perfectly fine with Gnome 3 on Jessie, only the actual UI element (button) is missing in the menu. My relative now uses the power button to hibernate, configured using gnome-tweak-tool. With the extension packaged it would be even better (https://bugs.debian.org/782741). Cheers