I haven’t had a chance to read the whole comic yet, but this is getting a lot of buzz online: there was an old Carl Banks Uncle Scrooge comic in which the Beagle Brothers infiltrated Uncle Scrooge’s dreams, using some sort of apparatus invented by Gyro Gearloose, to get him to reveal the combination to his safe. Gyro, Donald, and the nephews show up but can’t wake Scrooge and the Beagle Brothers for fear of serious mental repercussions, so they send Donald into Scrooge’s dream.
Uncle Scrooge: The Dream of a Lifetime
(Thanks to Walter Smith and Matt Staggs)
Update: Commenter Your Obedient Serpent says this is comic was by Don Rosa, not Carl Barks, and it dates from 2002 – which is a lot less impressive.