Wireless Multicast Forwarding with OpenWRT

We need to connect an STB device over wi-fi to the router. STB uses
multicast only. Using udpxy is not possible. Original firmware
forwards multicast over wi-fi using mac changing technique. I.e.
router changes destination mac address from multicast to unicast mac
address of multicast group subscriber. On Asus routers this feature is
called "Wireless Multicast Forwarding (WMF)". On Mikrotik it's called
"Multicast-helper". So, the question is: How I can  implement this on
TP-link wr741nd with ОpenWrt? It works in original firmware. How is it
implemented? Is it a feature of wi-fi chip driver or kernel patch or
patch of igmpproxy or something else?

igorv.sh@gmail.com