I have not seen any REST resource that operates on multiple documents at once, except for operations like clipping which obviously clip one document to another (but that's not really the same thing).
My impression has always been that batch operations are handled by the client, queueing them up and then calling the API request multiple times. When you do a batch operation on the web client, for example, you can see the progress gauge increment as each document is processed. Not sure it would look like that if it were a single API call to do a bunch of documents at once.
Is there an issue with simply creating a loop to process the documents in question? If you use the same ServiceConnection or HTTPServer request object, it should be pretty quick and only use up the license as it works. In a sense, having a single resource and just calling it multiple times is cleaner that having two resources, one for a single document and one for multiples. (Just my opinion...)