It should be possible to extend selected client-side GWT classes.

UPD It's now possible to configure deferred bindings in Widgetsets inheriting from PagesWidgetSet and MagnoliaWidgetSet.
There are some subtle hooks allowed by extending ComponentBar, AreaBar and ComponentPlaceHolder and overriding the selected methods. Keep in mind that extensions to the Widgetset tend to be expensive changes, and backward compatibility is not granted the same way as server-side components.