Ugh. After posting, I think I finally found it. If I have this right, it is in fact available via snmp using the following OID
SNMPv2-SMI::enterprises.9.9.171.1.2.1.1.0 (.1.3.6.1.4.1.9.9.171.1.2.1.1)
I am able to use this with Nagios as I wanted. I also did see a Health Event showing that the tunnel went down. Not sure why I wasn't getting the expected results before, so that is usable after all.
Mark