Technically, only frames listed in the vehicle builder chart are "official".  The line of reasoning against big, flying APCs was more a practical one than a balance one, i.e. in the terrain scale of a typical game (a typical table converst to 100 by 150 meters or so), it would be unlikely for large flying vehicles to have the kind of maneuverability that would keep them from "overshooting" the table during landing.

If your group is okay with it, there's no reason you couldn't take a basic (non-sprint, non-jump) APC or AFV frame and double its cost to make it a flier.