Add individual tag to widget title in sidebar

dynamic_sidebar_params is the filter that lets you modify those parameters on a per-widget basis. It fires for every sidebar though, so you’ll need to use both add_action & remove_action call. Try var_dump once before writing the code to get an idea of what you have to do

ADDED AN EXAMPLE ON REQUEST

This example adds an extra class to each widget depending on the order in which they appear i.e. first widget will have class “widget-1”, 2nd will have “widget-2” & so on

function add_first_last_class_to_widget($params) {
    STATIC $widget_num = array();
    $this_id = $params[0]['id'];
    $arr_registered_widgets = wp_get_sidebars_widgets();
    if (!isset($arr_registered_widgets[$this_id]) || !is_array($arr_registered_widgets[$this_id]))
        return $params;

    if (isset($widget_num[$this_id]))
        $widget_num[$this_id]++;
    else
        $widget_num[$this_id] = 1;
    $class="class="widget-" . $widget_num[$this_id] . ' ';

    $params[0]['before_widget'] = str_replace('class="', $class, $params[0]['before_widget']);

    return $params;
}