Thanks.
Just opened a Feature Request for that here : https://github.com/elasticsearch/elasticsearch/issues/1077
I will "try" to code it as writing rivers seems to be "easy"...
For cases where you can simply take the _source and index it into a new index (possibly with different mappings), then it makes sense. Having something built in elasticsearch for that does make sense, definitely. Its not a high priority since with the scan API, one can implement it on the client side.