I don't have an elegant solution, but it can be done. I'm assuming JSF 2+ & Facelets VDL.
For a managed bean Foo:
@ManagedBean @RequestScoped
public class Foo {
  private List<SelectItem> fooList = Arrays.asList(
            new SelectItem("value1", "label1", "description1"),
            new SelectItem("value2", "label2", "description2"));
  public List<SelectItem> getFooList() {
    return fooList;
  }
}
You can use JavaScript to set the title attribute on the DOM node:
<h:selectOneMenu binding="#{requestScope.fooSelectOne}">
  <f:selectItems value="#{foo.fooList}" />
</h:selectOneMenu>
<script>
(function() {
  var selectName = '#{requestScope.fooSelectOne.clientId}';
  var kids = document.getElementsByName(selectName)[0]
                     .getElementsByTagName("option");
  var index = 0;
  <ui:repeat value="#{foo.fooList}" var="_opt">
  kids[index++].title = '#{_opt.description}'; //TODO: escape this
  </ui:repeat>
}());
</script>