Can you use this to get Apple bonjour / mDNS working over a remote network (connected via VPN)? Or similarly, could you use it for a cloud seedbox to cast to a chromecast on your local network (via the VPN obviously)?
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.
Bridging L2 is not the optimal solution for either of your scenarios.