I liked the 6 hour window idea. It keeps people from attacking when everyone is not playing.
The problem I see with surprise raids is TFs and timed missions. Neither one is easy to go back to.
Also I think the notification system to the SG leader should be a @global message. Everyone plays alts and may not check on one for a while.
On another note, what about the SG coalitions? Can they raid/defend together?