How do I enable onboard (on screen keyboard) on the lock-screen?

I had this exact same issue. and no matter what I did I couldn't get it to work. So, on a whim I reinstalled Ubuntu from the latest version (14.10). I originally had 14.04 LTS. Nothing I did could get this to work (even reinstalling 14.04). So I downloaded the lastest release and did a clean install. And it looks as if the Devs fixed the issue in this latest release because now when I enable the option in Onboard settings, I get the keyboard pop up for logging in and from the lock screen. Hope this helps.

An additional note. When you enable the lock screen option you get prompted to enable Accessibility features. This is the key for log in screen. Onboard doesn't automatically pop up at login. But in the task bar, you'll see the little Accessibility icon (a little man figure in a circle). Touch that and you get the option to show the keyboard. This was also an issue with 14.04 LTS as it would never give me the accessibility icon in the task bar.