There is an option to advertise dns via the RDNSS option. See
http://linuxreviews.org/man/radvd.conf/ for more info on the options associated with it. I would like to be able to set the dns servers to be advertised over ipv6 links with radvd. I do not know how hard it is and I do not know where to submit feature requests so this seemed as good a place as any.