ReferenceError: Error #1069: Property hello not found on
_Main_mx_managers_SystemManager and there is no default value.
So I did the natural thing and Googled it before spending time figuring it out :)
Unfortunately, my search came up empty :(
Looking into the docs class mx.managers.SystemManager gave me the clue I needed. To get at the actual application, I needed to get to the first child of the loaded content. The compiler was unhappy with calling getChildAt() on the content object, so I cast it into an Object along the lines:
var flexApp:Object = (loader.content as Object).getChildAt(0);
That did the trick!