the problem is not the EL logic.

it is the Decode method in the UIComponent that decides when and where the ajax expand is made.
so I need to call it from within the UIComponent

I gave it a shot but it does not seem to work
maybe I'm doing it badly

key contains comma separated id's the different components are found.

public void render(FacesContext ctx,String key)
{ if (key==null) return;

 AjaxContext ajaxContext = AjaxContext.getCurrentInstance(ctx);

 Set<String> renderedAreas = ajaxContext.getAjaxRenderedAreas();
 if (key instanceof String)
 { String[] ids= ((String) key).split(",");//all the id's handle an encapsulation
 UIComponent par=this;//find the topmost parrent
 UIComponent ob;
 while ((ob=par.getParent())!=null) {par=ob;}//get the root parent
 for (int i=0;i< ids.length;i++) //for all id components
 { UIComponent comp=par.findComponent(ids);//find the components
 if (comp ==null) log.info("[ ajax upd component "+ids+" not found ");
 else renderedAreas.add(comp.getClientId(ctx));
 }

 }



}