The assignment of labels to addresses is an application feature built into the Web Demo, but it's not a feature of the MultiChain platform itself. In the Web Demo addresses self-publish their labels as individual items in the "root" stream. If you want to follow the same scheme there's no way to do mass assignment, since each address has to publish its own label in its own signed transaction. However you can of course do labelling in a different way, where one user is able to provide the label for many addresses using a stream in which both label and address are assigned as two keys to items with empty data payloads. In this case you could use publishmulti to publish a ton of stream items in one go.