For mDNS, you should run avahi to relay between subnets. For chromecast, that is SSDP/DLNA which is a multicast, so it is a matter of establishing mulitcast rou

For mDNS, you should run avahi to relay between subnets. For chromecast, that is SSDP/DLNA which is a multicast, so it is a matter of establishing mulitcast routing between sites. Bridging L2 is not the optimal solution for either of your scenarios.