Hi!
I am not really sure whether making all your civilians completely inactive is such a good thing. After all, by the time you get any real sieges, your fortress is a major operation with farming, mining, craftworking and who knows what going on. In addition, it usually covers quite a lot of ground so that even if the defenses were dented, dwarves working at the other end of the fortress would not be affected.
In addition, if something was able to breach your defenses thoroughly enough to reach your main business area, you are probably on the way out anyway.
So, I don't really see the benefit of paralyzing your entire fortress. After all, after a siege, your dwarves will also be busy collecting the loot and getting rid of the refuse, so in the end, you only cripple your fortress.
What would be interesting would be an improved versopn of the "Dwarves may not go outside" where you could designate a rectangular area as your fortress core. When the alarm is given, civilian dwarves may not leave the area of the fortress core and if they are not within it, they are to move there ASAP. This way, you could protect your civilians, have them out of the way of your soldiers and still keep business going. And you could place defenses in all directions with having a defense zone against the outside and against the demons on the inside, with the civilian area calmly between them.
Deathworks