Getting behind an instance portal can be extremely useful for PvP or for satisfying that undying curiosity.
Simply type /script SetDungeonDifficulty(3);
A message should appear saying something about entering Epic difficulty.
You can now walk freely through any and all instance portals.
Do undo this, type /script SetDungeonDifficulty(2); for Heroic or
/script SetDungeonDifficulty(1); for Normal.
Not posting screens, too lazy.
Enjoy!