I use NARHS, and perhaps it depends on your adviser, but we've had no problems with outside classes. I simply forward on the main components of work my ds completed for the class as well as whatever the instructor gave to us as proof of completion. That proof has been anywhere from a simple signed document (speech and debate and I actually assigned the grade) to a "report card" (Biology)
I would think that if you send in his work as well as whatever Great Books sends you as proof of completion that will be sufficient. I've never had them ask me to notarize anything.
Hope this was somewhat helpful ;)
Joy