Got it working!
Used these smcroutes in smcroute.conf:
mgroup from br-iot group 239.255.255.250
mgroup from br-lan group 239.255.255.250
mroute from br-iot group 239.255.255.250 to br-lan
mroute from br-lan group 239.255.255.250 to br-iot
and these custom firewall rules for TTL:
iptables -t mangle -A PREROUTING -i br-lan -d 239.255.255.250 -j TTL --ttl-inc 1
iptables -t mangle -A PREROUTING -i br-iot -d 239.255.255.250 -j TTL --ttl-inc 1
iptables -t mangle -A PREROUTING -i br-lan -d 224.0.0.0/4 -j TTL --ttl-inc 1
iptables -t mangle -A PREROUTING -i br-iot -d 224.0.0.0/4 -j TTL --ttl-inc 1
Thanks guys. Next hurdle is getting Spotify Connect to recognise devices across the subnets (it's possibly a mDNS problem)