Hi @downing,

New Quizzes definitely have an assignment_id (they are basically external_tool assignments).  The interesting thing is that in my testing, using the API list assignments call does not return any new quizzes though.  You have to use the new quizzes api to list the quizzes, and the id returned for each quiz is actually the assignment_id of the quiz, which you should be able to take and use to create assignment overrides as you seem to have done previously for other assignments

Let us know if this works, I just did some real quick and dirty trials here and everything seemed to come back as expected.

-Chris