ip mroute does not work, if
iproute2-3.8.0 and kernel 220.127.116.11 (hardened) is used
ip mroute rtnetlink answers: address family not supported by protocol - Google Search
Downgrade to iproute2-3.8.0
Maybe you should mask newer versions of sys-apps/iproute2 if you're inclined to keep using that old kernel. The older ebuilds are kept around for that reason.
I'd suggest to add a kernel-version check and warning message for ebuild info log. I.e. if installing 3.8 and kernel 2.6.32.x is active warn user that 3.3 should be used.
feel free to query on the kernel netdev list, but as Jeroen says, it's not generally a use case we care about. if there's a patch upstream has merged, then we can cherry pick it.