Here's my working smcroute.conf used to send SSDP through OpenVPN/IPsec:
mroute from tun0 group 239.255.255.250 to br-lan
mroute from tun1 group 239.255.255.250 to br-lan
mroute from eth0.2 group 239.255.255.250 to br-lan
SSDP packets need to have a TTL of 2 in order for multicast routing to work, achieved like this:
iptables -t mangle -A PREROUTING -d 239.255.255.250 -j TTL --ttl-inc 1
With this I am able to see Plex and Kodi UPnP/DLNA services on my iOS devices connected with either OpenVPN or strongSwan.