There's no built in SNMP support but you might want to check out the Event Notification Utility. You can read about it in the administration guide in the "maintaining Unity" chapter here:
http://www.cisco.com/univercd/cc/td/doc/product/voice/c_unity/unity31/sag/sag313/sag_0070.htm
it has the ability to trigger events on low disk, Unity becoming unresponsive (more to the point avCSMgr service not responding) and any other event that gets written in to the event log.
In the interest of full discosure, this little guy has had its share of problems along the way - most are fixed but it's earned a reasonably rough reputation for spiking the CPU in some scenarios and having to be disabled. But give it a look anyway...
