You should be able to, set the bool to 0:
SetOnStage(ITEMGUID_BLD_Humans_Prison_Grating_Door_A_000_b5e89243-b8ea-4873-99a5-371c713e32e2,0);
Correct -- using a script. There is no way to initially set it thru the Editor.
It seems then that we need an "initialization" type script that sets all items desired off stage.
This could be incredibly frustrating having to copy/paste perhaps dozens (hundreds?) or item GUIDs in a special script instead of simply marking the boolean directly in the editor.
For example, I'm currently working on a Keep wall. I want the wall to become breached by catapult attacks. So, I need to have 3 different wall segments w/breaches that need to be marked invisible, plus 3-9 different "rubble" items at the base of the breaches. I also likely need a dozen "catapult hit" locations randomly within the keep walls.
So, I've got to keep track of, and systematically, copy-n-paste at least 2 dozen items into a separate scene initialization script.
Please just expose, or let us somehow set specific booleans within the sidebar.