It is pretty easy on OPNsense but it's virtually impossible on many other types of routers. Meraki and Asus as two examples of that.

I'm actually running dnsmasq from a FreeBSD host. How would dnsmasq know about the self-configured addresses? I guess I could run DHCPv6, but it's next to impossible to get any kind of prefix delegation going with my ISP anyway. At the moment their router hands out the v6 addresses and I use v4 for dnsmasq.