You've probably read the "Getting behind instance portal" threads where you use: /Script SetDungeonDifficulty(3); for epic mode. Well with this macro correctly timed:
/Script SetDungeonDifficulty(3);
/Script ReloadUI();
You can go into (RFC for instance) with epic mode still on. Simply press that macro the split second before you zone in.
Here's a screenie of me in RFC while epic mode is still on, you can tell this because neither Normal or Heroic are check marked:
This should work with heroic mode:
/Script SetDungeonDifficulty(2);
/Script ReloadUI();
I haven't tested it though.
If anyone has any ideas of how this could be exploited into something useful, please mention them, thanks!