Since the metacourses allow normal users to manually enrol bug was fixed in Moodle 1.5.3, any guest user trying to access a metacourse which has been set to allow guest access with an enrolment key now get the error message This course does not allow public access, i.e. the same message a logged in user trying to manually enrol on the course would get.

As a bit of an aside I think this error message is slightly confusing for the user anyway, and would be much friendlier if it could read something along the lines of This course only allows access to students already enrolled on: (list of this metacourse's child courses).