Show Custom Comment Fields when editing in admin

Inserting a meta box in the Comment edit screen is the same as in the post types screens. But it can only be placed on the wide column, the sidebar seems not to accept other boxes.

To capture the data that’s being posted, I only found the filter comment_edit_redirect.

This has to be adapted to work with the plugin. The custom field in this example is meta_comment_field. See code comments:

// SAVE COMMENT META
// only found this hook to process the POST
add_filter( 'comment_edit_redirect',  'save_comment_wpse_82317', 10, 2 );

// META BOX
add_action( 'add_meta_boxes', 'add_custom_box_wpse_82317' );

/**
 * Save Custom Comment Field
 * This hook deals with the redirect after saving, we are only taking advantage of it
 */
function save_comment_wpse_82317( $location, $comment_id )
{
    // Not allowed, return regular value without updating meta
    if ( !wp_verify_nonce( $_POST['noncename_wpse_82317'], plugin_basename( __FILE__ ) ) 
        && !isset( $_POST['meta_comment_field'] ) 
        ) 
        return $location;

    // Update meta
    update_comment_meta( 
        $comment_id, 
        'meta_comment_field', 
        sanitize_text_field( $_POST['meta_comment_field'] ) 
    );

    // Return regular value after updating  
    return $location;
}

/**
 * Add Comment meta box 
 */
function add_custom_box_wpse_82317() 
{
    add_meta_box( 
        'section_id_wpse_82317',
        __( 'Meta Comment Meta' ),
        'inner_custom_box_wpse_82317',
        'comment',
        'normal'
    );
}

/**
 * Render meta box with Custom Field 
 */
function inner_custom_box_wpse_82317( $comment ) 
{
    // Use nonce for verification
    wp_nonce_field( plugin_basename( __FILE__ ), 'noncename_wpse_82317' );

    $c_meta = get_comment_meta( $comment->comment_ID, 'meta_comment_field', true );
    echo "<input type="text" id='meta_comment_field' name="meta_comment_field" value="", 
        esc_attr( $c_meta ), 
        "" size="25" />";
}