How to enable "Show message on Lock Screen" with Script
I'm an idiot! Just having the "LoginwindowText" key set to a string value is all you need!

My "problem" was two-fold:


The GUI won't display the checkbox as "checked" until you log out and back in.
I had a typo in the key name in some of my notes that I was copy-pasting from.


Hope this helps somebody else... I spent some serious time banging my head against a wall!