Usually REST backends handle single instance creation/update. You would need to change that to accept an array of objects.
That said, on the client side, you would need to go directly to the Backbone.sync function
Backbone.sync = function(method, model, options)
In this case your model should be an array of model. The method should be “create” or “save” and the options take the same type of options as a jQuery ajax call (error, success, etc.)