Johnny Castaway was made using the Dynamix Game Development System (DGDS), which was also used to create Heart of China and some Willy Beamish games. It’t be really great if that system would ‘leak’ somehow. Somebody’s gotta have it archived. Though no-one I co tacted so far seems to be that person.
About the rights: I guess you shouldn’t worry too much about them, since sierra clearly distantiated itself from JC, according to the accompanying readme file.