Well as there's no menu entry in the Gnome desktop anymore, I just typed "sudo hibernate-disk" into a terminal window. Alternatively, I can lock the Gnome screen, hit Ctrl-Alt-F1 to get to a pure text screen, log in there, and type the same. Or better, I there type "sudo hibernate-disk; exit". This is the only way I've found to get a password prompt on resume.
However, I also found that, nearly as often as not, the hibernate itself crashes. When it says "Writing image to disk..... 0%" it gets stuck at zero. Normally there it would count up to 100% and then switch the computer off. So, I'll have to switch off manually, and on switching back on, there's no resume but just a fresh boot. Should I file a separate bug report on this?