You’re correct it doesn’t support IGMP or any multicast control features, but it will forward multicast traffic as if it was a broadcast within the same broadcast domain (I.e. within the same VLAN).
I can’t say whether there is anything in development for the MX with regards to multicast, and Meraki is unlikely to say so in a public forum. I’d say there most likely isn’t, it’s more an MS feature, but you’ll need to check with your local AM/SE for a definitive answer.
