According to DD-WRT WIKI, only the Mega version or Big version DD-WRT supports OpenVPN and IPv6 at the same time.
Therefore, to get it prepared, it means you can source a DD-WRT supported router with 8MB RAM (or more), if you want OpenVPN and IPv6 to run concurrently on the DD-WRT router.
I believe it still takes some time to transit since
1) A significant number of working routers/equipment in Hong Kong can only support IPv4
2) In the mean time many ISPs are still in the stage of giving 6-to-4 mapping only