That might be, but then CalDAV/CardDAV would be reduced to WebDAV, so it would only be a WebDAV-Sync (because the server couldn't reply to CalDAV requests like "show me events between xx and yy"). As far as I have seen, Flock only "encrypts" the connection, say, is using SSL. But please correct me if I'm wrong!
What do you mean, "end-to-end encryption"? How should contacts/events be transferred encrypted from/to a CalDAV/CardDAV server? Those servers expected icals and vcfs, not encrypted data.