A little late, but it might help you and others, which where searching like I.
You first add under Firewallgroups a network with address 224.0.0.0 and the netmask 24. I named mine Multicast.
And then you add a rule under Firewallrules with the source firewall and select the blue network.
As destination you select the network, which was created in the first step.
Select accept. Save and activate the rule.
Depending on the services you need to add more rules from blue network, this time select the blue network from the standard networks as opposed to the firewall and as destination I selected the ip of my printer with the protocol tcp and the port 443. Some printers might need 631.
For airplay use the ip of the airplay device and add rules for udp 319:320 and tcp 65000:65535 and maybe you need the tcp 8000:8999 and tcp 40000:49999.
Sorry for my late follow up, Karl-Heinz
Your proposal sounds great and I’ll give it a trial as soon I’ve finished some other things.
Will keep you in the loop
BR
Thorsten