WordPress override bloginfo variable

For the example you mentioned, you can add a filter to the stylesheet_directory hook

add_filter('stylesheet_directory','change_stylesheet_dir') ;

function change_stylesheet_dir($stylesheet_dir, $stylesheet, $theme_root) {
  # do stuff - change the stylesheet directory
  $new_stylesheet_directory = 'http://mysites.com/hello/';
  return new_stylesheet_directory ;
 }

For any of the other parameters for bloginfo, you can look at the code for for bloginfo, which is in wp-includes/general-template.php. Follow the calls made for other bloginfo parameters to see whether there is a filter for it.