Wow! Good eye! You're right, that's definitely the same symbol. I wonder how/if they're tied together, and what the symbol represents.
My theory is the multi-point star represents the multiple paths one can take from the central hub (center of the star) or portal. In Riven, the hub is the beetle room, and the various paths or possible journeys are the different doors when the room is rotated properly. In Uru, the Nexus serves as a hub or portal, where you could link to different ages/locations. If that's the case, then I would think that there should be more instances of that symbol in the other Myst games, since they all basically have some sort of central hub or portal to other ages and locations.
sheesh....honestly, I didn't eat my Wheaties this morning!