Again, I'm not the ferry specialist, but I have heard, and it sound very logical to me, that if you need to ferry a kinda slower plane (a C-152 or C-172, say), from Canada to Greenland to Iceland, that there are specialist outfits in Canada which will outfit your aircraft with long-range ferry tanks in the cabin, and Transport Canada will grant you permission to take off something like 10 or 15% above MTOW. All that extra weight is fuel, to jack the endurance up to inhumane numbers.
www.earthrounders.com links to a large number of websites from people who have flown around the world (which invariably requires a North-Atlantic crossing) and they all refer to this kind of thing.