Twig/Extensionuser/Passwordmask.php を以下のように作成します

class Twig_Extensionuser_Passwordmask extends Twig_Extension
{
    public function getFilters()
    {
        return array('passwordmask' => new Twig_Filter_Method($this, 'passwordmask'));
    }
    public function passwordmask($string)
    {
        if (strlen($string) == 1) {
            return '*';
        }

        $out = '';
        for ($i = 0; $i < strlen($string); $i++) {
            if ($i + 1 == strlen($string)) {$out .= $string[$i];} else { $out .= '*';}
        }
        return $out;
    }
    public function getName()
    {
        return 'passwordmask';
    }
}

使い方

{{ 'aiueo123456' | passwordmask }}

表示結果

**********6