Preliminary support for OpenVPN is now provided for the purposes of creating and using TAP devices on multiple platforms. OpenVPN is compiled in as another host module, with a special tap leaf interface, that uses the same options as OpenVPN itself. See the OpenVPN docs for further information.
In particular, it can be used to configure tap interfaces with IP or IPv6 addresses. It can also be used to run postconfiguration scripts that can, e.g., set up NAT using the filter rules appropriate to the platform. Here is a sample line that can be used to accomplish these:
tap0 at le0: tme/host/openvpn/tap --dev tap --ifconfig 10.0.77.1 255.255.255.0 --ifconfig-ipv6 "2001:db8::1/64" "2001:db8::2" --tun-ipv6 --up nat.sh
Note that this is currently the only way to get networking support under Windows.