That settings isn't currently exposed in LuCI, it seems it got lost a while back, will look into it later.
For now you can specify a static IPv6 address like this via SSH:
uci set network.wan.ip6addr=fdca:1:2:3:4::1234/64
uci set network.wan.ip6gw=fdca:1:2:3:4::1
uci commit network
ifup wan