As I wrote: it's simple to untag multiple VLANs on a single port. E.g. if there are 3 VLANs with multicast streams with VLAN IDs 100, 200 and 300 ... and you have fourth VLAN for other IP communication of said device (e.g. management) with ID 999, then you would configure a bridge like this:
/interface bridge
add name=bridgeMCAST vlan-filtering=yes
/interface bridge port
add bridge=bridgeMCAST interface=<uplink interface> ingress-filtering=yes frame-types=admit-only-vlan-tagged
add bridge=bridgeMCAST interface=<untagged client interface> ingress-filtering=yes frame-types=admit-only-untagged-and-priority-tagged pvid=999
/interface bridge vlan
add bridge=bridgeMCAST tagged=<uplink interface> untagged=<untagged client interface> vlan-ids=100,200,300
add bridge=bridgeMCAST untagged==<untagged client interface> vlan-ids=999
The last line is kind of redundant as interfaces with pvid set are automatically added as untagged ports to corresponding VLAN ID.
Mind that the example above is incomplete with regard to management access to device (you would have to either add a LAN port to bridge with same pvid where you would connect management station. Or something in this line.