GWT 2.5 will add a UiRenderer that reuses the UiBinder infrastructure to build "renderers" from a ui.xml file, in the aim of building Cells.
That's probably not what you were looking for though (declaring the Cell to be used from within the ui.xml rather than havein to use @UiField(provided=true) or a @UiFactory), and I'm not aware of any changes being done about this.