It is actually more constrained than that since other instances of
the application might be running on non-multi6 aware hosts.
So the conservative approach would be to always hand an IP
address/locator
to unmodified applications, that way the application can use it
for callbacks and referrals without any constraints on which hosts the
different instances of the app is running.
For a transition scenario this will most likely more be the rule than
the exception. SIP comes to mind of an application where you would do
referrals and you can not trust that all "nodes" are of the same
"instance"/protocol version/awareness.