If you didn't already, take a look at this article that should be really useful to you: StageAspectRatio Enhancements in AIR 3.3
You could of course play with setAspectRatio to define starting orientation, but that's only part of the problem.
You can modify the autoOrients property located in your app descriptor file if you want to get rid of the automatic behavior and handle everything manually. Or you could listen for every orientationChanging event of the Stage object and preventDefault() them to cancel any screen orientation.
Once this is done, you simple have to handle the loading orientation of your views and this should be it.