Offloading Uploads to FTP Location


Does anyone know how to change the uploads location in multisite to an FTP location? I've heard that, in non-multisite, you can enter an FTP hostname and credentials in the uploads directory field. But, of course, multisite is different.

The goal is to upload files directly to a CDN, rather than to store them on our local server (taking up space) and then have them copied to the CDN. Since they'll be served from CDN anyway, there's no need to waste space on the server with images just sitting there.

Any idea how to set an FTP location as the upload directory, and how to create the necessary sub-directories there as needed?