Use the ReverseProxy option for that. Use the URL you would normally use to connect to the Synchronet webserver, like
VirtualHost {
...
ReverseProxy .* http://<Synchronet hostname>:<Synchronet port>/
}
Hiawatha is able to communicate via IPv6. Simply don't use the Interface option in a binding (to make it bind to all IP address on that interface) or specify an IPv6 address.