Could be possible that UNIT_HEALTH_FREQUENT may fire correctly even if you are not targeting the unit. On the other hand I'm pretty sure UNIT_HEALTH_FREQUENT is nothing more than a hidden OnUpdate in the back...
Same idea is used here:
http://code.google.com/p/isler-wowui...=svn71&r=71#19
Gonna test it. If it works adding "self.Health.frequentUpdates = true" should fix the issue on bossframes.