This is done with code used in the beforeFactory script:
des = reportContext.getHttpServletRequest().getParameter("__designer");
if( des == null || des == "false" ){
mytable = reportContext.getReportRunnable().designHandle.getDesignHandle().findElement("mytable");
mytable.setProperty( "dataSet", "viewer" );
}
<br />
Remember to name your table to mytable.