I'm honestly not sure off the top of my head (haven't looked at Zenoss code in about 6 months).
Zep Facade (to see how events may be queried):
http://dev.zenoss.org/trac/browser/trunk/Products/Zuul/facades/zepfacade.py#L114
Zep Service (to see what Google Protocol Buffers are returned):
http://dev.zenoss.org/trac/browser/trunk/protocols/python/zenoss/protocols/services/zep.py
EventFilter (Google Protocol Buffers for events):
It might be easier to write a ZenPack that makes use of the above information if the JSON API doesn't support what you're trying to do.
Hope this helps,
Tim.