Of course, just because there won't be any major hardback books about Kara-Tur, Maztica, or Zakhara doesn't mean they don't peek through here and there, albeit rarely. While this is no substitute for having major books on those regions, it does confirm that at least they aren't trying to retcon them out of existence.
In Underdark there's a nest of stingers (scorpionfolk apparently) who managed to get to the Underdark of Faerun from over in Maztica. They have some large plans for the region, and maintain their worship of their Maztican gods.
I can't recall anything specific for Zakhara that we've seen so far (save for that issue of Dragon; see below).
In Unapproachable East, there is a section on the region with Shou expatriats, and several towns that have a large amount of Shou culture. There's even a small bit on the Golden Way (the path that leads from Faerun to Kara-Tur).
And of course, Dragon issue 315 featured materials for all three of those campaigns (as well as several other old ones). Al-Qadim had the sha'ir written up as a core class. Kara-Tur Oriental Adventures had a list of ancestor feats by region, along with a great description of the nations of the continent. And Maztica had two prestige classes for two different organizations of "knights" over there.