Patrick (aka kaber) has just made available his work on memory mapped netlink. Both the kernel and the libmnl part are available on git.kernel.org.
You can pull kernel code other net-next tree:
git pull git://git.kernel.org/pub/scm/linux/kernel/git/kaber/nl-mmap-2.6.git
Libmnl code can be fetched:
git clone git://git.kernel.org/pub/scm/linux/kernel/git/kaber/libmnl-mmap.git
Once done a NETLINK_MMAP kernel compilation option is then available via make config.
Documentation is available in the Linux tree. It is in the file: Documentation/networking/netlink_mmap.txt