The release version of Myst was a Hypercard stack, too, at least on the Mac. If you changed the type/creator on the right data files, you could open them in Hypercard. The images wouldn't display without some extra jiggery-pokery, but I do recall being able to view the scripts. Hypercard didn't support any sort of source code encoding, so the scripts were out there in the open.
I mean it would probably be taken down in a heartbeat, because the disclosure of the source code was very likely unintentional, but, you never know!