In The Deathly Hallows, Neville reveals how he managed to create a tunnel between the Room of Requirement and Hogsmeade:
"It's quite straightforward, really," said Neville modestly. "I'd been in here about a day and a half, and getting really hungry, and wishing I could get something to eat, and that's when the passage to the Hog's Head opened up. I went through it and met Aberforth. He's been providing us with food, because for some reason, that's the one thing the room doesn't do.
Inspired by this comment: could Draco Malfoy have created such a tunnel in The Half Blood Prince? Could he have simply thought "Man, this vanishing cabinet stuff isn't working out. I really wish there was another way for the Death Eaters to waltz into Hogwarts" and then a tunnel would be created?