My opinion is this- if you're going to make a legendary item, there need to be legends about it. One of my pet peeves is that often you learn about the "lore" behind an item immediately after you get it, like there was a carfax report taped to bottom of it or something. A legendary item should have a history attached to the area it's found in, and you should be able to run across mentions of it fairly easily when you're in that area. Then when you find the quest to get it or fight the boss who has it or encounter the riddle it's hidden behind, you'll know what you're looking to get.
My way of implementing this into the game would be a "rumors and stories" option at the various inns and taverns you encounter along the way. Mixed in with more mundane things like main story info and side quests would be tidbits about items you could find in the general area and where you might go to find out more.
By the way, people hoping these things will be rare may be disappointed. From kickstarter rewards alone, there are going to be at least 57 player-created items, nevermind the developers themselves.