I've got the import via API working now - the imported questions go into a new "No Name" question bank, and a new quiz is automatically generated. I can rename that new quiz to something appropriate to the import (that poorly-named quiz is a bug in the web interface too), but unfortunately there is no Question Bank api available for me to move questions into the new quiz
SAD
My next attempt will be to skip the QTI migration step - just take the quizzes I have in a 3d-party format and import them directly using the Quiz/Question API. As I represent an organization transitioning into Canvas, these activities are high priority.