I'm currently using a custom-compiled openvpn with Viscosity. IPv6 (using a tun device!) works like a charm, I only had to add "tun-ipv6" in the settings (and of course configure the server appropriately).
Given that OpenVPN 2.3 alpha 1 will shortly be released (Samuli Seppänen is hoping this or next week):
What is the status for IPv6 support in Viscosity?