Assuming you're using Mustache.php v2.1, you can use the LambdaHelper passed as an optional second argument to your helper for rendering the original block body:
$mustache->addHelper('_curstyle', function($text, $mustache) {
switch($mustache->render($text)) {
case 'RUR':
return 'Russian rubles';
case 'USD':
return 'US dollars';
case 'EUR':
return 'Euro';
default:
return $text;
}
});