How to theme a radio button field on node edit form?
I think what you need to do is just altering the form array. You can accomplish this in your own hook_form_alter implementation. 

For example:

function YOURMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
    if($form_id == 'YOUR_CONTENT_TYPE_node_form') {
        $form['YOUR_ELEMENT_NAME']['#attributes']['class'][] = 'your-custom-class';
    }
}