Using a permissions hook is a good way to control write access with no user logged in: http://docs.elgg.org/wiki/Permissions_Check

o.k. "elgg_set_ignore_access(TRUE)" is a quick way to unlock the permissions

after looking to cash's hint . . . and for flexibility, transparency and best practice i did it with a permission hook

on the page, where i would like to add my new object (without user login)
i set the context to:  set_context('addsupport');

in start.php of my pluggin i add this:

   // override object permissions
    register_plugin_hook('permissions_check','object','object_can_edit')

//  Override the canEdit function to return true for object within a particular context.

  function object_can_edit($hook_name, $entity_type, $return_value, $parameters)
   {
   if (get_context() == 'addsupport')
   {
     return true;
   }
   return $return_value;
   }

tnx cash