For those of you who are looking for a way to integrate natural sorting with the UCA rules this hack seems to work:
<?php
$array = array
(
    '1', '100',
    'al', 'be',
    'Alpha', 'Beta',
    'Álpha', 'Àlpha', 'Älpha',
    'かたかな',
    'img1.png', 'img2.png',
    'img10.png', 'img20.png'
);
echo '<pre>';
print_r(sortIntl($array, true));
echo '</pre>';
function sortIntl($array, $natural = true)
{
    $data = $array;
    if ($natural === true)
    {
        $data = preg_replace_callback('~([0-9]+)~', 'natsortIntl', $data);
    }
    collator_asort(collator_create('root'), $data);
    return array_intersect_key($array, $data);
}
function natsortIntl($number)
{
    return sprintf('%032d', $number);
}
?>
Output:
Array
(
    [0] => 1
    [1] => 100
    [2] => al
    [3] => be
    [4] => Alpha
    [5] => Beta
    [6] => Álpha
    [7] => Àlpha
    [8] => Älpha
    [9] => かたかな
    [10] => img1.png
    [11] => img2.png
    [12] => img10.png
    [13] => img20.png
)