KnockOff with Nova - solved
For those wanting to use Nova Launcher and still retain the double tap 'KnockOff' feature, I finally got it working via the procedure below. Note that this requires the paid version of Nova.. Hope this helps someone!
1. Download "Turn off the Screen" from the play store (it is free)
2. Open the installed app and click "Enable Now"
3. Open up Nova Settings -> Gestures and buttons -> Double tap -> Shortcuts -> Activities
4. Scroll to "Turn off the Screen" and expand the options. Choose "Turn Off The Screen : LockerActivity"
That's it. You should now be able to click anywhere on the nova desktop without an app / widget and it will turn off the screen as the stock LG KnockOff does. Note that I began with the "Screen Off and Lock" app, but this ignored the LG setting to not lock the screen for 10min and forces immediate PIIN entry upon unlock.