Comment 15 for bug 1307163

I think I've found a work-around.
Open dconf-editor.
Search (ctrl+f) for "screen-keyboard-enabled".
Turn it on.
(I have both org.gnome.desktop.a11y.applications and org.cinnamon.desktop.a11y.applications. I'm not sure which you need.)

http://bazaar.launchpad.net/~unity-team/unity/trunk/view/head:/lockscreen/LockScreenController.cpp
http://bazaar.launchpad.net/~unity-team/unity/trunk/view/head:/lockscreen/LockScreenSettings.cpp
The Unity LockScreenController falls back to a org.gnome.ScreenSaver dbus proxy when settings->use_legacy, which is set to true when either the screen reader or the on-screen keyboard is enabled.