• You have to allow 1900/udp with destination address 239.255.255.250 from both networks
• You have to reflect mDNS between the networks (which means more ports)
• Update: you need to enable 8008-8009/tcp as well

I'm using nftables but you should get the idea:

  # This is obviously not a complete example...

  define homeassistant_if = eth0
  define chromecast_if = eth1
  define homeassistant_host = 192.168.0.2

  chain firewall_out {
    type filter hook output priority 100; policy drop;
    # ...
    oifname $chromecast_if udp dport 5353 ip daddr 244.0.0.251 ct state new accept
    oifname $homeassistant_if udp dport 5353 ip daddr 244.0.0.251 ct state new accept
  }

  chain firewall_in {
    type filter hook input priority 0; policy drop;
    # ...
    iifname $chromecast_if udp dport 5353 ip daddr 244.0.0.251 ct state new accept
    iifname $homeassistant_if udp dport 5353 ip daddr 244.0.0.251 ct state new accept
  }

  chain chromecast_forwarding {
    type filter hook forward priority 0; policy drop;
    tcp dport { 8008, 8009 } ip saddr $homeassistant_host ct state new accept
  }

  chain chromecast_out {
    type filter hook forward priority 0; policy drop;
    # mdns
    udp dport 5353 ip daddr 224.0.0.251 ct state new accept
  }

  chain homeassistant_out {
    type filter hook forward priority 0; policy drop;
    # ssdp
    udp dport 1900 ip daddr 239.255.255.250 ct state new accept

    # mdns
    udp dport 5353 ip daddr 224.0.0.251 ct state new accept
  }

Update: The scary part here is that I have no idea why it's enough to just enable 8008-8009/tcp in the forwarding chain for the chromecast network..? I should really try to understand why that works…

This is fairly simple as well, install and enable avahi on your machine, do some basic configuration:

  # /etc/avahi/avahi-daemon.conf - but just the important changes
  [server]
  allow-interfaces=chromecast_if,homeassistant_if
  [reflector]
  enable-reflector=yes

I setup a FreeBSD jail¹ in each subnet and installed avahi-app on both. You can either use the nss_mdns adapter (and then host(1) or ping(8)) or avahi-browse to test it. I've only had partial success with the nsswitch.conf changes so I got lazy and just used avahi-app (rather that reading the manual).

  # pkg install nss_mdns

  # /etc/nsswitch.conf
  # Add mdns to the end of the hosts line, like this:
  hosts: files dns mdns

  # or, if that doesn't work:
  # pkg install avahi-app
  # service dbus enable
  # service dbus start
  # service avahi-daemon enable
  # service avahi-daemon start
  # avahi-browse -a --ignore-local --terminate

If you can use ping to resolve .local addresses, you should be fine. If the system configuration doesn't like using mdns, try avahi-browse instead and see if you list non-local hosts with it. It should return a list that looks something like this:

  # avahi-browse -a --ignore-local --terminate
  + epair0b IPv4 testwifi                                      _ssh._tcp            local
  + epair0b IPv4 testwifi                                      _sftp-ssh._tcp       local
  + epair0b IPv4 CCCC12312312@Audio+                           _raop._tcp           local
  + epair0b IPv4 CCCC23423423@SHIELD+                          _raop._tcp           local
  + epair0b IPv4 googlerpc                                     _googlerpc._tcp      local
  + epair0b IPv4 Chromecast-Audio-$uuid                        _googlecast._tcp     local
  + epair0b IPv4 SHIELD-Android-TV-$uuid                       _googlecast._tcp     local
  + epair0b IPv4 SHIELD                                        _nv_shield_remote._tcp local
  + epair0b IPv4 SHIELD                                        _androidtvremote2._tcp local
  + epair0b IPv4 $uuid                                         _googlezone._tcp     local
  + epair0b IPv4 Homeassistant-instance-name                   _home-assistant._tcp local
  + epair0b IPv4 HASS Bridge xyzyxzyxz                         _hap._tcp            local
  + epair0b IPv4 homeassistant [longstring]                    _workstation._tcp    local
  + epair0b IPv4 router [00:ma:ca:dd:r]                        _workstation._tcp    local

This Works For Me™ and might help someone else. I can now poll and control my Chromecast devices from my Home Assistant installation.