don't know whether i really got ya but you will need to put the modalpanel on the same page where you want it to appear on top. you can then call it with the respective richfaces/js function (id of your panel as parameter).
You either put the source for the modalpanel into MyOpenerFile.jsp or you INCLUDE it (jsp:include)
You can then use Richfaces.showModalPanel. Please not the LOWERCASE "f" in "Richfaces".
I also made that "mistake" first, because the documentation was simply wrong (copy/pasted).
<h:form> <h:commandButton value="showPopup" onclick="Richfaces.showModalPanel('myPopup')" </h:form> <jsp:include src="MyPopupModalPanel.jsp"/>
<rich:modalPanel id="myPopup"> <h:form> .... </h:form> </rich:modalPanel>