Ok. I figured it out. The Example Configurations page says:
> NOTE: When behind a firewall appliance that passes protocol 41, use the IPv4 address you get from your appliance's DHCP
> service instead of the IPv4 endpoint you provided to our broker.
when I removed the "local" line entirely, ifup'd and ifdown'd the interface and started radvd, things worked again.
Phew. I'm crazy about IPv6 and use it all over the place, even my printer is addressed by ipv6. So I'm lost without it.