unity's lock screen has no way to force a log out on Trusty

[jdreed]

The functionality simply does not exist. A possible workaround might be to write an indicator applet, give it a desktop_lockscreen profile, and have it display whatever menus we need, but there's still the question of how to end the session gracefully. See /usr/share/unity/indicators for an idea of what the indicator service files look like and the various profiles they ship. This uses D-Bus menus with Gio.Menu and Gio.Action, and is horribly undocumented.