Hey James,
I just found this MIB OID in an old email from 2013:
etsysNacApplianceMIB=1.3.6.1.4.1.5624.1.2.73
Below this tree you should find:
Authentication Statistics:etsysNacApplAuthenticationRequests .1.3.6.1.4.1.5624.1.2.73.1.1
etsysNacApplAuthenticationSuccesses .1.3.6.1.4.1.5624.1.2.73.1.2
etsysNacApplAuthenticationFailures .1.3.6.1.4.1.5624.1.2.73.1.3
etsysNacApplRadiusChallenges .1.3.6.1.4.1.5624.1.2.73.1.4
etsysNacApplAuthenticationInvalidRequests .1.3.6.1.4.1.5624.1.2.73.1.5
etsysNacApplAuthenticationDuplicateRequests .1.3.6.1.4.1.5624.1.2.73.1.6
etsysNacApplAuthenticationMalformedRequests .1.3.6.1.4.1.5624.1.2.73.1.7
etsysNacApplAuthenticationBadRequests .1.3.6.1.4.1.5624.1.2.73.1.8
etsysNacApplAuthenticationDroppedPackets .1.3.6.1.4.1.5624.1.2.73.1.9
etsysNacApplAuthenticationUnknownTypes .1.3.6.1.4.1.5624.1.2.73.1.10
etsysNacApplAssessmentRequests .1.3.6.1.4.1.5624.1.2.73.1.11
etsysNacApplCaptivePortalRequests .1.3.6.1.4.1.5624.1.2.73.1.12
etsysNacApplContactLostSwitches .1.3.6.1.4.1.5624.1.2.73.1.13
etsysNacApplIPResolutionFailures .1.3.6.1.4.1.5624.1.2.73.1.14
etsysNacApplIPResolutionTimeouts .1.3.6.1.4.1.5624.1.2.73.1.15
etsysNacApplConnectedAgents .1.3.6.1.4.1.5624.1.2.73.1.16
Give it a try and run it agains your NAC appliance.
Kurt