Does it have to be via web-interface?
From console/ssh-session it is doable without reboot by adding something like this to /etc/network/interfaces and doing ifup on the new vmbr interface. After that you see the interface in the web-console and can bind vm's nics to it.
auto vmbr777
iface vmbr777 inet manual
bridge_ports eth0.777
bridge_stp off
bridge_fd 0
