Upload media to different sub-domain
I'm not 100% sure of what your goal is, but I just accomplished something similar and in case it helps you here is what I was looking to do and what I did.

I am uploading all images to the /sites/default/files/images directory and I wanted them to become available via http://images.example.com/filename.jpg and display on the node via ths subdomain.

To do this I setup a virtual host in /etc/httpd/conf/httpd.conf

&lt;VirtualHost *:80&gt;
  ServerName images.example.com
  DocumentRoot /var/www/html/example.com/sites/default/files/images
&lt;/VirtualHost&gt;


You can then alter your subtheme node.tpl.php to reference the attached images via this subdomain.

&lt;?php $img = 'http://images.example.com/' . $field_image['und'][0]['filename']; ?&gt;
&lt;img src="&lt;?php print $img; ?&gt;" /&gt;


I wanted the convenience of having the variables already present in the node so I used the template_preprocess_node(...) function in template.php.

function MYTHEME_preprocess_node(&amp;$vars) {
  $node = &amp;$vars['node'];
  if ($node-&gt;type == 'image') {
    $base_url = 'http://image.example.com';
    $vars['image_base_url'] = $base_url;
    foreach ($node-&gt;field_image['und'] as &amp;$image) {
      $image['url'] = $base_url . '/' . $image['filename'];
    }
  }
}


With this I can access inside the node both $image_base_url as well as $field_image['und'][0]['url']


$image_base_url is http://image.example.com
$field_image['und'][0]['url'] is http://image.example.com/filename.jpg