It doesnt take that long to recap valleys and make new NPC's unless they arent the level you want them to be yet. If you can. drop and recap after maintanance then its no issue. If thats not an option, and your worried that others will port in, just NPC them. Although that would be a real waste. Take the lesser of two evils.
As Morgan said, this is also happening to your opponents. They are prolly just as worried maybe not in the same way but maybe for farming reasons. If they have such a big army that they could port in and do some damage, they need a way to feed that army.
