Okay, you are getting into an area which I have no experience. So take this only as a suggestion.
ZS uses OpenVPN and looking at http://openvpn.net/index.php/open-source/documentation/miscellaneous/76-ethernet-bridging.html it seems that the bridge setup is on the “server” side of the VPN. That is the ZS side.
It might be that you can just do the server side part of the bridging setup from the tutorials on the ZS site…