Awesome. The Arduino port "IPv6 support" issue was closed 9 days ago too. No other IPv6 related issues reported yet. Off to look for reports from elsewhere of people using it...
I only found one example for the Espressif-provided RTOS, at https://github.com/IPv6-ESP8266/IPv6-ESP8266 . The example says it only works in a IPv6-only network and it's using link-local addresses, which kind of defeats the point of IPv6. No idea if these reflect limitations of the stack. It's a start at least :)