Doing more research, it looks like the recommended method for handling this situation is to include the router preference options in the Router Advertisement. From https://tools.ietf.org/html/rfc4191#section-5.1:
The best router for the default route is the router with the best
route toward the wider Internet. The router least likely to redirect
traffic depends on the actual traffic usage. The two concepts can be
different when the majority of communication actually needs to go
through some other router.
For example, consider a situation in which you have a link with two
routers, X and Y. Router X is the best for 2002::/16. (It's your 6to4
site gateway.) Router Y is the best for ::/0. (It connects to the
native IPv6 Internet.) Router X forwards native IPv6 traffic to router
Y; router Y forwards 6to4 traffic to router X. If most traffic from
this site is sent to 2002:/16 destinations, then router X is the one
least likely to redirect.
To make type A hosts work well, both routers should advertise
themselves as default routers. In particular, if router Y goes down,
type A hosts should send traffic to router X to maintain 6to4
connectivity, so router X and router Y need to be default routers.
To make type B hosts work well, router X should advertise itself with
a High default router preference. This will cause type B hosts to
prefer router X, minimizing the number of redirects.
To make type C hosts work well, router X should in addition advertise
the ::/0 route with a Low preference and the 2002::/16 route with a
Medium preference. A type C host will end up with three routes in its
routing table: ::/0 -> router X (Low), ::/0 -> router Y (Medium),
2002::/16 -> router X (Medium). It will send 6to4 traffic to router X
and other traffic to router Y. Type C hosts will not cause any
Note that when type C hosts process the Router Advertisement from
router X, the Low preference for ::/0 overrides the High default
router preference. If the ::/0 specific route were not present, then a
type C host would apply the High default router preference to its ::/0
route to router X.