Hi Richard,
If you are on Zenoss 4.x then when you create a Command template there should be a dropdown for JSON in the COMMAND data source dropdown. It is new with 4 though.
The only "documentation" that I know of is a ticket (now closed) that defined what needed to happen - http://jira.zenoss.com/jira/browse/ZEN-2633 .
Cheers,
Jane