The idea that we need a backup federal government is frightening. If, God forbid, every Washington politician were killed in an atomic blast, the Constitution would still be the law of the land and order would subsequently be restored. Frankly, I don't see how getting a fresh start in Washington could be at all bad for this country. (Yes, I'm being somewhat sarcastic.)
What about regional/national networks? And how long would "subsequent" be? Who would be running the Department of Defense, Border Patrol, etc., during "subsequent"? Philosophically, what you're saying sounds nice, but not very realistic. I'd also like you to show me where the Constitution forbids it.