Takayuki Miyoshi on "[Plugin: Contact Form 7] Custom shortcode in emails"

Try this:

add_filter( 'wpcf7_special_mail_tags', 'your_special_mail_tag', 10, 3 );

function your_special_mail_tag( $output, $name, $html ) {
	if ( 'quick-flag' == $name )
		$output = do_shortcode( '[quick-flag]' );

	return $output;
}

Place [quick-flag] where you wish the flag.