It could work with S3 ( take a look at W3 Total Cache which can mirror uploads onto S3), but the issue would be security (it would no longer be authenticated with each request; I could just send the URL).
The smarter way would be to mount the bucket to an EC2 instance, and set the document upload directory to that bucket. That way, the documents are stored in S3, but requests are still authenticated.
Hope that helps,
I was curious if anyone had tried using something like Amazon S3 for actually storing the documents? I was comparing this to http://wp-client.com/. I much prefer the features of this plugin because of the security and document revision management, but the one feature that I like about wp-client is it's ability to configure remote document storage with Dropbox or S3.
Thanks!