Yeah that'd be interesting to try. If you extend the endpoints and the vocabularies around RSS/OPML enough, you could likely create a fairly robust dataset replication protocol. Microformats are a decent schema basis, but you might want to build upon them as well. If you want to make the users' sites reusable as a datastore across applications, you could introduce an oauth flow to enable 3rd party writes. You might also want to add content signatures so you can verify the data authenticity from third parties (e.g. to support the equivalent of reposts).