Martin,
One thing that you may be able to take advantage of is the Notification Engine in NAC. It can be triggered by multiple events and conditions can be applied that are based on the current state of an end system and any rule components. Based on those events and conditions being matched, multiple actions can be taken. Some of those actions include a Syslog message, SNMP Trap, Email, or a custom script being executed.
This may be a method that you could use to send information from NAC to an external service.
Tyler