WordPress: Apply filter/hook to a particular sidebar widgets?

If you were to add var_dump($params); to the top of your callback you’d notice that it contains references to both the sidebar name and the sidebar ID. You can use those to control when this runs, provided you know the name or ID.

function widget_params( $params ) { 
  if ('Main Sidebar' === $params[0]['name']) {
    // do something
  }
  return $params;
}
add_filter( 'dynamic_sidebar_params', 'widget_params' );