We would really like this option too.
We're in Bali, Indonesia and most guests come from different time zones (i.e. Europe is 7 or 8 hours earlier).
It would be great if I could write my replies when I wake up in the morning (which is the middle of their night) but schedule them to be sent during my afternoon, when they're more likely to be awake.
I want to keep all correspondence on Airbnb's site for convenience, so I'm not interested in using an outside company for this.
Cheers