Thanks for the suggestion. It looks like this is only a feature of Moodle 1.9, as Moodle 1.8 doesn't have the option for a 2nd parameter for the rebuild_course_cache function.
But, I actually tried to reproduce the hidden assignment error in my local instance of Moodle 1.9.2+, and I was able to create Turnitin Assignments successfully even with hidden assignments, so I'm guessing something changed on the Moodle end and caused this error not to happen.
So, I guess this is an issue for those using pre-Moodle 1.9.
Any other ideas?