Comment 14 for bug 857813

Another solution may be to have a gsettings vendor override file for gnome-screen-saver. That way Onboard wouldn't need to run once to have lock screen working. I don't believe this should be installed by Onboard though, pretty sure not by the upstream version and probably neither with an Ubuntu patch.

Anyway, I believe it would be named
/usr/share/glib-2.0/schemas/org.gnome.gnome-screenshot.gschema.override

and look like this
[org.gnome.desktop.screensaver]
embedded-keyboard-command='onboard --xid'
embedded-keyboard-enabled=true