Hello,
I'm currently working on the following scenario:-
Two routers, R1 and R2. Both running different versions of Openwrt.
R1 is a wired router between the internal network and the Internet.
R2 is a wireless router connected to a LAN switch port on R1.
R1 and R2 both provide DHCP services to their respective LANs for different subnets.
A DLNA DMR (renderer) is connected to another LAN switch port on R1.
A DLNA DMS (server) is connected to a LAN switch port on R2.
Both of these devices use the multicast address 239.255.255.250 UDP port 1900 (SSDP) to discover other devices.
Available interfaces on R2 are eth0.2 (WAN) and br-lan (LAN, including wireless clients).
I would like to repeat these multicast packets received from eth0.2 to br-lan and vice-versa.
I have installed smcroute and tcpdump onto R2.
So far, I have managed to get R2 to join 239.255.255.250 on the WAN and repeat packets to br-lan, which I have observed using tcpdump.
The opposite direction (R2 LAN -> R2 WAN (R1 LAN)) however doesn't seem to work.
Admittedly I've never used smcroute before and I'm not certain if I am configuring it correctly.
Does anyone have experience setting this up? 
I know that SSDP packets from R2 will contain a LAN address for the server, but I can fudge that by adding a static route on R1 for that address to go via R2.