It is not possible to send a message through OpenERP livechat to all livechat users. In fact, this is not something you should want to do considering all portal users (external to your company) are also potential livechat users. It means that anyone who receives an email with a link at the bottom and signs up to see their document in your instance will create a user. In other words, any lead or any supplier could be in the livechat.

If you are looking for a group chat capability, you should use groups in the Messaging application and have the right people in the right group. In your example, you could have all the employees working in a certain office location in a group and that's how you would announce that lunch has been served.
Bon Appetit.