Matthew Jakeman [off-list ref] writes:

I am pondering attempting to write a module that will allow me to
intercept an IPv6 packet once it has been fully constructed (so just
before it gets passed to the output interface). The point of this is
so that I can play around with the packets construction however I
please to try out a few things.

You need a netfilter module on the OUTPUT chain.  For further
questions on that please ask the netfilter mailing list.

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only.