Index: modules/nodereference/nodereference.module
===================================================================
--- modules/nodereference/nodereference.module  (revision 793)
+++ modules/nodereference/nodereference.module  (working copy)
@@ -719,6 +719,8 @@
   foreach ($references as $key => $value) {
     $options[$key] = $value['rendered'];
   }
+  drupal_alter('content_allowed_values', $options, $field);
+
   return $options;
 }

Index: modules/userreference/userreference.module
===================================================================
--- modules/userreference/userreference.module  (revision 793)
+++ modules/userreference/userreference.module  (working copy)
@@ -630,6 +630,8 @@
   foreach ($references as $key => $value) {
     $options[$key] = $value['rendered'];
   }
+  drupal_alter('content_allowed_values', $options, $field);
+
   return $options;
 }