Nagios is a powerful service monitoring tool that is easily extensible using scripts and programs of nearly any language. So we decided to write a short bash script that helps us to monitor the ZLM zen_queue entries with it.
That short script uses a local postgresql client, connects to the zlm data store (postgresql by default) and queries all entries of it. It counts all entries by the different types and compares the "New" state entries against a warning and critical value as well as the "In Progress" actions against a limit.
Based on that values the check program reports the state of the queue.
With this solution we are able to get a notification when there is something going wrong and we do not have to wait till the managed devices by zlm recognize some problems.