How I've addressed the transaction issue in the past is to expose it specifically as a transaction. For instance, POST /posts/transactions would contain a list of transformations to /posts and will receive a transaction id as a response. The id can be polled for success (/posts/transactions/:uuid) or results can be pushed to a browser. It is more work to be able to roll that back if one fails, but that's a coding issue not so much an API one.

I'd love some feedback on this approach. How do other people allow efficient mass changes?