This little requirement must be the worst snag in the game. And the worst thing is, it's so obvious once you have the answer. How could Low possibly know how to activate the tomb entrance without having first seen it in the map room?
Incidently, I just solved this problem for someone else on another forum. How many others encountered this? I only recall it from the second time I played through, I guess because I already knew that the tomb should open without needing to see the map.
First time surrealists are often confused by the similarities between fish and telephones.