if your OpenVPN is already connecting two or more IPv4 LANs its very easy to add IPv6.
just have a look on IPv6 Payload Patch for OpenVPN 2.1 and 2.2 for the new options.
get your own ULA at sixxs.net for the internal IPs of the OpenVPN server and client.
add following to your server.conf
server-ipv6 fd77:76c1:d29b::/64 push "route-ipv6 2001:db8:161:2461::/64" push "route-ipv6 2001:db8:161:2ffa::/64" route-ipv6 2001:db8:6f:217::/64 route-ipv6 2001:db8:58ac::/48
- 2001:db8:161:2461::/64 and
are your networks on server side,
are your networks on client side and
is your ULA.
you also need to adapt the clients configuration in the client configuration directory on the server ccd/your.client
iroute-ipv6 2001:db8:6f:217::/64 iroute-ipv6 2001:db8:58ac::/48
restart your server and your IPv6 networks are connected via the OpenVPN.