How to add class to individual radio item inside "radios"?
No there's nothing built-in for this (take a look at form_process_radios(), nothing in there that would let you specify individual classes).

No need to hack core though like in the solution you linked to. Simply implement hook_element_info_alter(), and replace the process function with your own, which will be based on the original form_process_radios(), but that can handle individual classes for options.

function MYMODULE_element_info_alter(&amp;$info) {
  // You might want more advanced logic here, to replace instead of override altogether,
  // in case other modules have already altered the core info.
  $info['radios']['#process'] = array('MYMODULE_process_radios');
}

function MYMODULE_process_radios($element) {
  // ...
  return $element;
}