I did wonder if this could be done via CSS, but it looks like
that isn't [yet] possible.
Subsequently, I poked about in the PHP for the data plugin. I couldn't see anything which looked like it would do what you want, but you might try editing this bit of helper.php:
default:
$val = $this->_addPrePostFixes($column['type'], $val);
//type '_img' or '_img<width>'
if(substr($type, 0, 3) == 'img') {
$width = (int) substr($type, 3);
if(!$width) {
$width = $this->getConf('image_width');
}
list($mediaid, $title) = explode('|', $val, 2);
if($title === null) {
$title = $column['key'] . ': ' . basename(str_replace(':', '/', $mediaid));
} else {
$title = trim($title);
}
if(media_isexternal($val)) {
$html = $R->externalmedia($mediaid, $title, $align = null, $width, $height = null, $cache = null, $linking = 'direct', true);
} else {
$html = $R->internalmedia($mediaid, $title, $align = null, $width, $height = null, $cache = null, $linking = 'direct', true);
}
if(strpos($html, 'mediafile') === false) {
$html = str_replace('href', 'rel="lightbox" href', $html);
}
$outs[] = $html;
} else {
$outs[] = hsc($val);
}
Specifically, you could pass this line through a regular expression replacement to format numbers in the way you want:
$outs[] = hsc($val);
...but this is a hack, and certainly not best practice. I don't have time to test it (sorry) but it might work...?