Sort of. Technically it's a NativeWindow which is maximizable or not, not the app itself. A NativeWindow is created as maximizable or not via NativeWindowInitOptions, and it can't be changed after it's created. Now, when you setup the "default content" window options with AIR, the runtime is basically creating the first window for you as a convenience, with those settings. However, there's nothing stopping you from making your initial content nothing more than a hidden window which will create a new window as maximizable or not. At this point, it would not be hard to re-create your main window with different maximizable values. (This is also how I implement multiple-instance like apps, since only 1 AIR app can be running at a time.)
Hope that helps!
-Aaron