Some time ago JuanJo wrote some patches to use OpenVPN via IPv6-links. i use the patches permanently under gentoo, debian and OpenWRT.
Here is a patch for the current in trunk-version of freetz included openvpn-2.1_rc21. The patch includes:
- changes on the openvpn-options-webpage to activate IPv6
- the IPv6-patch 0.4.10 (from http://github.com/jjo/openvpn-ipv6/downloads)
- an enhancement for "make menuconfig" to choose whether to include the patch or not based on ipv6-capability of freetz-image
It would be fine, if the patch is included in the trunk. For future updates only the file 001-ipv6_support.patch in "patches/cond/" needs to be updated with the current from the link above.