按照@Rarst关于安全字符的提示，我最终做了如下工作：

function wpa_105883_menu_title_markup( $title, $id ){
    if ( is_nav_menu_item ( $id ) && ! is_admin() ){
        $title = preg_replace( \'/#BR#/\', \'<br/>\', $title );
    }
    return $title;
}
add_filter( \'the_title\', \'wpa_105883_menu_title_markup\', 10, 2 );

function wpa_105883_menu_title_markup( $title, $id ){
    if ( is_nav_menu_item ( $id ) ){
        $title = str_ireplace( "#BR#", "<br/>", $title );
    }
    return $title;
}
add_filter( \'the_title\', \'wpa_105883_menu_title_markup\', 10, 2 );