Give rsync the ability to write/read its stream to/from an archive file. Almost certainly the best option would be for rsync to maintain its own archive file format. Why do this? Because rsync is awesome, we have learned all of its options by now, and unlike tar, cpio, etc., rsync captures all metadata, and rsync streams across a connection. A future enhancement would be the ability to append an update to the end of the file. Another enhancement would be optionally to append an index, so that files could be retrieved quickly from the archive. This enhancement could be useful for versioned backups.
Is --write-batch or --only-write-batch what you are looking for? If it is and you are trying to do backups I suspect you probably want rdiff-backup instead. But then you will be back here eventually for performance reasons.
Yeah, the batch stuff already allows streaming of the protocol, which can batch up some changes into a single file. It doesn't allow appending, and really I think we should not complicate it further.