Hi Nick,
If I understand correctly you are proposing what I already described in my previous comment.
Using openDatabase() with CREATE_IF_NECESSARY before passing "PRAGMA cipher_migrate", migrates the DB successfully.
From your answer it seems that CREATE_IF_NECESSARY is needed so that the temporary DB is created.
I think it would be a good idea for the migration guide to be updated, to point that this flag (CREATE_IF_NECESSARY), or that the openOrCreateDatabase() method are required for the migration to succeed.