GWT Composite best practices
I'm not sure if I understand what you are trying to do. But for all the Composite's I've written I've never overridden the initWidget method. Because Composite itself doesn't need to be initialized with a constructor, i.e. no need to call super() my constructors of widgets extending composite look something like:
public mywidget() {
SomePanel p = new SomePanel();
....
initWidget(p);
}
As a best practice, imo, only the widget extending Composite should call it's 'own' initWidget.
"GWT Conference: Best Practices for Building Libraries" gives a couple of tips. You should also look at the source of GWT and at the source of one of the libraries for GWT (like gwt-ext)
[EDIT] I just saw another option: suco. From the description:
A micro library that helps to maintain your GWT client code clean and modular.