have you considered using rabbitmq/0mq? It sounds like they are perfect candidates for what you are trying to achieve.
If you have 100 users, each one waiting for a message in their queue, then you can "broadcast" a single message to all of the queues (or a group) with one command. All the users waiting for a message in their queue will get a copy.
Another approach which is hack-ish imo, is to use something like jabberd to broadcast messages.