Even if you could get the guest login to work, you would not have an individual record of identified to the individual, so Instead of using the "guest" login. Why not make a generic login with student access to the course. So you would make a new user, maybe Guest2 and assign it to the course. Make the quiz unlimited numbers of tries. Put this information on the login page. EX:
If you are a guest wanting to take the introductory quiz, please use Guest2 as the username and 12345 as the password to login.
If you did want to know who had taken the instance of the quiz, you could put in a quiz question asking for the persons name and perhaps the email address.
Paula