I'm facing the same issue and did a little of investigation. I settet up a server and tried two DNS configurations: AAAA only (does not work) and A & AAAA (works). Using the hostname in the "Join IP" dialog seems only to trigger a DNS A lookup, so currently IPv6 seems just not to be enabled.
Also I could not figure out a format to enter the IPv6-Address into the "Join IP" dialog.
Would be nice if the Iron-Gate people could provide intel on this matter.