on the versatility of network-bridge

Network-bridge, the default Xen network backend, doesn't care what sort of data it's sending -- as far as it's concerned, they're Ethernet frames.  There's a special case for IPv4 and the antispoof rules, but that's it.  Other protocols, like IPv6, will "just work," and there's no current provision for Xen to inspect packets.  (Although it wouldn't be that hard to add, building on the IPv4 support.)