Census data for mercenaries excludes Level, which makes it difficult for us to determine which equipment slots have been unlocked for a merc. There should be a LEVEL value somewhere here: http://census.daybreakgames.com/s:eq2wire/xml/get/eq2/mercenary/?c:sort=ts:-1
This is not a bug, but instead it is a feature request. Any additions to this system will need to have a longer conversation as its impacts are internally far reaching.