As a reference, this is how I extended the open method as per @john-macintyre's suggestion:
$.widget( "ui.dialog", $.ui.dialog, {_x000D_
open: function() {_x000D_
$(this.uiDialogTitlebarClose)_x000D_
.html("<span class='ui-button-icon-primary ui-icon ui-icon-closethick'></span><span class='ui-button-text'>close</span>");_x000D_
// Invoke the parent widget's open()._x000D_
return this._super();_x000D_
}_x000D_
});
_x000D_