you may use standard binding to create it dynamically.

As for calling - its just JS functions calls so you may cal them in any handler you need.

please (a very simple example):

page:

<h:form>
 <a4j:commandButton value="showMp" oncomplete="javascript:Richfaces.showModalPanel('_panel',{left:'auto', top:'auto'})" action="#{mpBean.mpInit}" reRender="test"/>
 </h:form>
 <a4j:outputPanel id="test">
 <rich:modalPanel binding="#{mpBean.mp}"/>
 </a4j:outputPanel>

import javax.faces.component.html.HtmlOutputText;
import org.richfaces.component.html.HtmlModalPanel;

public class MpBean {
 HtmlModalPanel mp = new HtmlModalPanel();

 public MpBean() {
 }
 public String mpInit() {
 mp.setId("_panel");
 HtmlOutputText text= new HtmlOutputText();
 text.setValue("Hello");
 mp.getFacets().put("header", text);
 HtmlOutputText text2 = new HtmlOutputText();
 text2.setValue("sample");
 mp.getChildren().add(text2);
 return null;
 }
 public HtmlModalPanel getMp() {
 return mp;
 }
 public void setMp(HtmlModalPanel mp) {
 this.mp = mp;
 }
}

How do you actually show the modal panel from the backing bean?

Suppose you have a method in the bean, and that method returns and error, and you wish to show that in a modal panel. So you want to open the modal panel from within the backing bean method.