David, 

Please give up, now and forever, this idea of hardcoding kernel
policies based on conflating IPv6 semantics on the one hand, with
RFC-1918 semantics on the other.  That is not an acceptable approach,
for reasons explained elsewhere. Not least because it's a half-arsed
kludge that may work for *you*, with your mix of addresses; but will
fail for other people forced to use, for example, multiple distinct
non-globally-routable addresses.

Try intsead to articulate what you *actually* want.  The best approach
would be an exact example, with specific globally-routable IP
addresses and subnets.  Clearly distinguish which of the IP addresses
on a multihomed machine you wish to be "first-class" addresses, and
which are to be ``second-class'' addresses.

Then we can come up with some reasonable design, which works for *any*
multi-homing setup, with *any* pcombination of globally-routable
and/or non-globally-routable subnets.