Roles and permissions

Well, this is one way to do it but I don't know if it is the most efficient.

1. At the site level, assign the role of guest to the user(s) who you want to have view access to certain courses. (Note: be sure to click the checkbox to hide this role assignment from other users.)
   
   Administration » Users » Permissions » Assign system roles
   
   
2. In the courses that you want to open up for viewing to the user, select the "Allow guest without the key" option under that course's settings.
   
   
3. All other courses should be set to "Do not allow guests in" under course settings.

Since your site is setup to not allow guests, individuals will not be able to login as a guest at the site level, and the Login as guest button will not appear. However, the user you will have designated as a guest site-wide, is already an Authorized user with the role of guest. This user will be able to enter any course that allows guest access but none of the others.

Note: Your teachers and administrators will see this user listed under their participant lists in each course as a hidden participant, as they have a site-wide role designation; however, the user will not be able to enter or view these other courses.

I just tried this out on my own site, and it seems to work.