Access hostvars from a group of hosts in a play

Lets say you need to perform an action and include hostvars from a group of hosts. Information on how to do this in templates can be found here: http://ansible.cc/docs/playbooks2.html#magic-variables-and-how-to-access-information-about-other-hosts - but what if you want to do this during a play?

An Example:

You have a controller system that needs to register all hosts in a particular group as slaves for the application. In this example, the group is named "slaves" and the hostname and ip address of each host is required.

name: Register slave systems
action: command register-slave --hostname ${hostvars.{$item}.ansible_hostname} --ip ${hostvars.${item}.ansible_em1.ipv4.address}
with_items: ${groups.slaves}

As seen, this allows you to iterate through a groups hostvars for use in your play!