This is ambitious. I once had a college class where the students were very upset because I decided to change the number of in-class quizzes from 5 to 4 a few weeks into the course. (The quizzes made up 10% of the overall grade.) Students hate it when you do anything even remotely weird or unexpected with assessments. Telling them that there is going to be a mystery trick assessment will just make them anxious and grumpy.