“What would it be like for the developers of a major franchise game (Zelda, Castlevania, Metroid, etc) to put in a LOT of hidden content that was SO difficult to find/required so much technical skill at the game that most players would never get to play it, even with the use of FAQs?”
A true Icebergvania has a lot of hidden content - so much that one's entire perception of the game changes when you find it. These fellows don't get that far.