I am not sure I understand the question right. Yes, you can use EL in the reRender and assign the value anywhere prior the RENDER RESPONSE phase.
The potential problem I see in your code is not about the reRender itself, but about rendered attribute on the textarea.
Try to avoid pointing with reRender to the components that has 'rendered' attribute. As an alternative, you can surround such component with < a4j: outputpanel layout="none" >

Sorry, I don't get it. I've tried:

<a4j:log/>
 <a4j:poll action="#{bean.updateStats2}" immediate="true" interval="5000" reRender="gamesRow"/>
 <h:panelGrid>
 <a4j:repeat binding="#{bean.gamesRepeat}" id="gamesRow" value="#{bean.gamesIt}" var="games_it" ajaxKeys="#{bean.ajaxedRowsSet}">
 <h:panelGrid columns="2">
 <h:panelGrid>
 <h:outputText id="valA" value="#{games_it.valA}"/>
 <h:outputText id="valB" value="#{games_it.valB}"/>
 </h:panelGrid>
 <h:panelGrid>
 <h:outputText id="valC" value="#{games_it.valC}"/>
 <h:outputText binding="#{bean.valDText}" id="valDText" value="#{games_it.valD}"/>
 </h:panelGrid>
 </h:panelGrid>
 </a4j:repeat>
 </h:panelGrid>

public List ajaxedRowsSet() {
 ListDataModel model = (ListDataModel)gamesRepeat.getValue();
 TempModel row = null;
 int rows = model.getRowCount();
 List list = new ArrayList();
 for(int i=0;i<rows;i++) {
 model.setRowIndex(i);
 row = (TempModel)model.getRowData();
 System.out.println("id: " + valDText.getId());
 list.add(valDText.getId());
 }
 return list;
 }

sorry, I have posted my message in wrong thread... ;)