Comment 31 for bug 528094

Since you have disabled the gnome-screensaver, the dbus object org.gnome.ScreenSaver is not created while the Gnome starts.

We need to create the dbus object org.gnome.ScreenSaver and implement the Lock method.

I use the below python script to create the org.gnome.ScreenSaver.

----------- myscreen-dbus.py --------
#!/usr/bin/python

import dbus
import dbus.service
import dbus.glib
import gobject
import os

class ScreenDbusObj(dbus.service.Object):
    def __init__(self):
        session_bus = dbus.SessionBus()
        bus_name=dbus.service.BusName("org.gnome.ScreenSaver",bus=session_bus)
        dbus.service.Object.__init__(self,bus_name, '/org/gnome/ScreenSaver')

    @dbus.service.method("org.gnome.ScreenSaver")
    def Lock(self):
        os.system( "xscreensaver-command -lock" )

if __name__ == '__main__':
    object=ScreenDbusObj()
    gobject.MainLoop().run()

You should keep this script running on the background. The 'Lock Screen' button will call the method Lock and the method Lock will execute the xscreensaver-command -lock.

To automatically start this script, you need to add the command, i.e. /path-to-the-script/myscreen-dbus.py & in the System/Preferences/Sessions.

P.S.
You don't need the ScreenSaver .service file