While NetworkManager recieves RDNSS neighbor discovery user option from kernel, it doesn't recieve DNSSL at all. This can be debugged with NetworkManager
(or hopefully some better testing tool) and radvdump (to check if DNSSL is present).
radvdump reports DNSSL is there, NetworkManager gets no netlink message from kernel.
your best bet for networking bugs is to contact the upstream developers directly.
It looks like even 3.4 lacks support for DNSSL, I quickly scanned through the diff.
I hoped I would clean up the list of IPv6 bugs a little bit before World IPv6 Launch:
My posting to firstname.lastname@example.org:
Kernel 3.5 will bring DNSSL support.
Fixed in 3.5. Using daily. Works.