..remember whether it is possible to override the "part of base plan" (movable) for doors/windows/staircases with the FurnitureLibraryEditor...

Did a quick check, and I do not think it is possible to override. Once a piece of furniture is set to Door/Window, the movable option appears to be ignored.
[font=courier new]movable#2=true
doorOrWindow#2=true[/font]
.. and the item cannot 'opt out' of the base plan. See thread 1781.

ok