OS is Debian sid. BIND 9.7.3.
It has no ACLs (except whatever's the not-overridden-in-config-file default), and has "listen-on-v6 { any; };" (as per Debian's default).
IPv6 is supported (ssh, smtp, http[s] and imaps all running on IPv6 sockets). Ah, yes, it is dual-stack (of course).
BIND says "No IPv6 interfaces found" but, obviously, there is one.
Pilot error (and a very misleading error message): I had a "-4" in the options in /etc/default/bind9 (don't recall why I put it there, but I recall now that I did so)
The misleading error is that Bind reports not finding an interface when I expressly told it not to do IPv6. It should report the latter. Which it doesn't.
So, problem solved.