Well, override does not make sense for constructor. You can always override the constructor. Effectively you always do as if you don't supply a constructor a default one is created for the class, which then calls every other constructor in the hierarchy, effectively overriding them.
One thing that AS is missing that you see in many other object-oriented languages is abstract base classes - a glaring omission as there are many times you want a base class that is meant only for use as a derived instance. DisplayObject
In both the classic API and Starling for example.
So I wonder if that is what you are dealing with, some code that implements abstract base classes - it certainly sounds like it. I have not had to deal with this myself but you can look at starling.display.DisplayObject for a straightforward way to do it – in the constructor and in a function that must be overridden.