What you are trying to do can be done RESTfully. Although POST is commonly used to add a single entity to a collection, it can also be used to submit a chunk of data to a "data processing resource". This pretty much opens the door to do anything you want with a POST.
People frequently misunderstand the REST constraints and believe that you MUST use PUT and DELETE in order to be RESTful. This is not the case. There is no requirement to use all of the methods. The only requirement is not to misuse the ones you do use.
Obviously it is going to be more intuitive to a user of your API if you POSTed a single entity to a collection to create an entity. However, if this is not a viable option for you then you are free define a new media type that can contain a set of objects to be created/updated by some data processing resource.
The important thing is to be explicit about these rules. You should create a new media type, like application/vnd.yourcompany.objectlist+xml and you need to write documentation about how that media type should be structured.
Now that you have defined the media type your client is going to need to know where to POST that list of objects. Ideally you would define a new link relation (e.g. objectprocessor) that can be used in other documents to provide an URL to POST to. In the specification document of that Link relation you should explain to client developers, that when you post an application/vnd.yourcompany.objectlist+xml to a link that has the link relation objectprocessor then what is going to happen is x, y, and z.
Assuming your root URL returned some kind of XHTML document like:
<html>
<h1>My awesomely restful service that can create objects in batches</h1>
<link rel='objectprocessor' href='http:/example.org/myservice/objectprocessor'/>
</html>
A client application could process this document, discover this link and understand based on the link relation definition that it needs to POST a application/vnd.yourcompany.objectlist+xml to the endpoint specified in href.
In this way, the message is completely self-descriptive, you are interacting with resources identified with URLs that are discoverable and you are within the rules of the uniform interface.