Hi Emnoc,
the issue is (as always) with wireless bridging which is desirables for quite a few users. If multicast hits the internal interface I feel the wireless might get unusable in the situation.
My goal was actually stupid enough (but only for testing & curiosity anyway). I have an IPTV setup at home and am at the point of plugging the IPTV into the Fortigate which is my only switch in the living room. the other rooms are connected through an IGMPv3 capable switch and are " protected" .
Still it should be simple to listen for IGMP messages and enable / disable ports in my opiniont. multicast forwarding has all the IGMP logic built in and the ports are fake anyway.
So the way to go is a) buy another switch or b) don' t do wireless bridging.