Required OpenWRT package is 'python-light'.
However it is missing some prerequisites that argparse requires. Make a directory called '/usr/lib/python2.7/encodings' and copy the contents of 'openwrt-python-encodings' into it.
One parameter is required for multicast-relay.py to run on OpenWRT:
--homebrewNetifaces
For certain OpenWRT architectures, you may also need to set --ifNameStructLen for multicast-relay to be able to correctly determine interface names. The default is 40, but you may have more luck setting it to a different value:
--ifNameStructLen=32
Note that only interfaces that have IPv4 addresses configured may be used as parameters to --interfaces. If you have bridges configured then you will probably need to specify the bridge names and not the underlying interface names.