Available since version 1.6, this filter allows you to customize or insert HTML strings after main tag, before rendering bricks data.
add_filter( 'bricks/content/html_after_begin', function( $html_after_begin, $bricks_data, $attributes, $tag ) {
if ( $tag !== 'main' ) {
return $html_after_begin;
}
// Insert custom div after the main tag
$my_additional_html = '<div class="my_notification">This is my notification</div>';
return $html_after_begin . $my_additional_html;
}, 10, 4 );