Currently, vmadm doesn't support setting IPv6 addresses on zones. A workaround is to set allow_ip_spoofing on the interface and configure it manually inside the zone. For example:
(Re)Boot the zone and log into it. Inside, create temporary address objects for IPv6 and (probably) a default gateway:
You should now have IPv6 connectivity, assuming the addressing used aligns with the actual addressing on the NIC the zone uses. The changes will not be persistent so this needs repeating after reboot.
A script together with an SMF service can make sure that happens. For example, this can be used to automatically set an IPv6 address with address based on the current IPv4 address;