The idea of Airgroup is two things:
Proxy multicast requests so that users across subnets can see devices advertising services
Allow the administrator to turn on drop broadcast and multicast on the virtual ap and still support multicast DNS across subnets.
The problem with multicast DNS is when it is seen in the air, not necessarily on the wire. If you turn on drop broadcast and multicast on the virtual ap AND turn on air group you should be fine.