IGMP snooping is a layer 2 feature used within a switched Ethernet network. It is not related to routing multicast traffic from one subnet to another across the interfaces of a router, which is layer 3 - which is done with the help of an IGMP proxy, usually. (The proxy forwards the group membership queries from the clients upstream to the IGMP querier, so that the upstream network knows to forward specific multicast streams to the user)
If a consumer broadband "router" has IGMP snooping it is because it has a multi-port layer 2 switch on the LAN side and that IGMP snooping feature applies to layer 2 forwarding of multicast between the LAN ports, (just as it does with a normal ethernet switch) not forwarding between WAN and LAN which is done with an IGMP proxy.
Two completely different things. Keep in mind that most consumer broadband routers are a Layer 3 router with a Layer 2 ethernet switch hanging off the LAN side of that router to provide multiple Ethernet ports. So they can have both Layer 3 and Layer 2 features. (The PFSense router I'm using does not have any built in Ethernet switch however, just a number of discrete ethernet ports, and it is only plugged into the upstream modem and into a 24 port switch on the LAN side)
Not that it's probably any use to me as I don't want phone but I've just realised PFSense can also run a PPPoE server, and I have four discrete ethernet ports available on the hardware running PFSense.
So in theory I could connect PFSense directly to the ONT with WAN configured as a PPPoE client, connect the LAN port to my normal network as it does now, and also configure one of the unused Ethernet ports as a dedicated PPPoE server expecting the same, known credentials the BT Hub 2 is wanting to use to connect upstream, and connect only the Hub to that port.
This would give the BT Hub 2 the PPPoE connection it is expecting. Assuming the right kind of authentication can be configured on the PPPoE server it should at least connect and get some degree of connectivity.
Whether this would be enough to give full functionality would depend on whether it would be happy being behind another NAT device, as it would get assigned a private IP address and anything behind the Hub would be double NAT'ed.
But it might be enough for the digital voice service and/or the IPTV service to work. (Maybe with IGMP proxy/multicast forwarding firewall rules...)
If I do end up switching I could certainly see if I could get the Hub to connect via PFSense and at least pass normal internet traffic through the hub.
You can certainly connect directly to the ONT with your pfsense with a PPPoE connection. You will NOT however, be able to use it for Digital Voice.