Here are some great examples of these articles explaining how to deploy a DMZ agent:
http://pkjayan.wordpress.com/2010/05/17/agent-managed-untrusted-servers-step-by-step-guide/
http://blogs.technet.com/b/stefan_stranger/archive/2012/04/17/monitoring-non-domain-members-with-om-2012.aspx
There are even some homemade scripts to deploy the agents like this one: http://systemscentre.blogspot.com.es/2012/03/scom-dmzworkgroup-agent-deployment.html