Quagga supports adding additional IPv6 prefixes to the router advertisements, via:
ipv6 nd prefix 2001:db8::/64
It would be nice if NCLU supported this, so I didn't have to resort to hitting vtysh directly:
cumulus@cumulus:~$ sudo vtysh -c 'configure t' -c 'interface swp1' -c 'ipv6 nd prefix 2001:db8::/64'
Docs here: http://www.nongnu.org/quagga/docs/docs-multi/Router-Advertisement
What I'm actually doing here is adding a static route at an interface:
net add routing route 2001:db8::/64 swp1
Then wanting to send router advertisements with that prefix.