This has been around for many years now, but it seems that there are hardly any implementations. I extended one of my projects back then with RemoteStorage support to allow users to upload files to their preferred storage site (rather than my limited server), but there were only 1.5 public sites where people could upload stuff.

Originally, ownCloud had a plugin for RemoteStorage support, but none of the users colud make it work for their ownCloud instances. It was a rather frustrating time. The project still offers a RemoteStorage connection, but I expect this feature to bit-rot away eventually.

Pity.