My suggestion would be to setup the uploads directory as a remote symlink (using NFS or something similar) so that the files go directly to the backend system when you upload them to the website.
But that's what I'd do. Anything else will require you changing the Wordpress upload code. It's all the stuff in upload*.php in the wp-admin directory. You can modify it to work or it's possible to replace it entirely without any modification (via a plugin). Your call.
