The case is that I dont' want to recompile the original library. So I tried to use hxswfml to create a SymbolClass for the necessary class - very ugly, and didn't work ;) -.
I know that there are other ways to override code in a static library, still, I think its strange that hx project code does not overrule the as3 static linked code - I believe its always desirable that user code overrides linked libraries, which is, correct me if i'm wrong, the case for pure haxe projects.
All in all I'm starting to believe that is bug in the swf compiler backend...