I wonder if you did an snmpwalk on a Windows server in this configuration if you'd find some OID that talked about its primary/standby status like you do with HSRP on Cisco routers. It'd be worth a look.
If you have APM I wonder if you might use WMI to monitor a cluster class? Here's a link to Microsoft about that class http://msdn.microsoft.com/en-us/library/aa372876(VS.85).aspx
Also, I think there's a CLUSTER.LOG file which may be monitored with APM. I don't know that for sure, but if I learn more then I'll pass it on.