CDN, SEO and image styles: 301 on GET request
I dont really understand your question, but we handled a similar problem.
We also did rewrtie for all images to static server (different server running lighttpd).
On the lighttpd the files folder was mounted as well, so all files are in sync. 

We also had a domain that forced back to one of the webservers (varnish>drupal), and not to use the CDN server . 
If the file did not exist in the static server it got rewrtitten (temporary) back to the webserver, which created the imagecache file.
From that moment, the file was there and it would always be there.

attr = lighty.stat(lighty.env['physical.path'])
 if (not attr) then
 lighty.header["Location"] = "http://directto.webserver.com"  .. lighty.env["request.uri"]
 return 302
end


Hope this can help.