Updating this question to say that as of Android 4.0, IPv6 is fairly well supported. There are still no visual indicators that it is running/working, but devices pick up IPv6 addresses and try to use them by default.
Android 5.1.1 supports IPv6.
Here you han a screenshot of the console ‘netstat’ command (https://twitter.com/juan_domenech/status/732535218662412288).
You can also check this typing ‘what is my ip’ on Google and observing whether you have an IPv4 or an IPv6 coming back (only in IPv6 WiFi networks obviously).
Turns out that DHCPv6 is not necessary after all (SLAAC might be in place here).