I don't think it's possible out of the box (via an XMPP request from a client). It would also be some kind of information leak, if everybody could discover all users.
You could solve it via plugin and use your own "protocol".
E.g. we use it to get the roster count for a user. We send a IQ query to a plugin (AbstractComponent) which returns the roster count for a given user. We use a custom IQ extension for this.
Alternatively, you might want to read / try AdHocCommands. I think they are used for such cases. But I am not familiar with them.
In any case, I believe you have to write your own plugin.