Actually, this requirement is supported by our new feature. This feature is essentially the last of your suggestions, ie. "custom file system extension", except that it's built-in and is configurable via the manager.
Due to an oversight, HTTP was actually left out of the configuration dialog for gateways in version 6.2.0, but it will be available in 6.3.0, which is due out next week. If you would like to experiment with it before then then please e-mail us for a link to a preview. I tested it just now to make sure that it works for your case and it worked fine. It took only a minute to set up.
That's the good news. The bad news for you is that the gateway feature is only available in the enterprise edition, but we may be able to offer you a discount. Again, please e-mail us for more information.
- Hans (EnterpriseDT)