There is no IPv6 for the ReadyNAS NV (v1)
Question: what firmware update will supply it.
Answer: no such update will ever exist.
Reference: IPV6 support for NV+; By mdgm; In the ReadyNAS Community Forum; 2012-07-04.
Therefore: avoid the ReadyNAS NV+ (v1, sparc)
Stick with ReadyNAS “Ultra” or “Pro” (v2, x86, Atom)