Controlling file paths in multi domain setup
I use the sledgehammer approach and search/replace from template_process_html:

function foo_process_html (&amp;$variables)
{
  $variables["page"] = str_replace("http://internal.example.com/", "/", $variables["page"]);
}


You can also get fancy and use preg_replace if you need better control.  I typically end up needing this when I move a site from our staging server, which is a subdomain off of ours, onto a client's own domain.

If I were in your shoes, I would think about having an internal theme and an external theme, combined with one of the choose-a-theme-based-on-domain-name solutions.  Then you can add preprocess/process as needed to just the external theme.