You just mention the 2 methods that are candidates for the translation of a string. If you do not want to fork the project nor change the localized files then I believe there is no other way of translating it.
Last resort method: Make an identical file of django-registration
that includes the to-be-translated string and add translations there.
IMO the only way is to fork the project, run ./manage.py makemessages
and voila! Translations are there. Another thing you can do is to try to contribute to this package by fork it first and then make a pull request! That's the beauty of open sourcing!