First off I'm not really sure you guys will consider this an exploit or not, I for one think it is because it involves getting around limitations set by Blizzard by using clever mechanics. Hence being an exploit.
Okay so really this exploit is not strictly for Halls of Reflection, it can be used with any instance where you can regain entrance after you died, but can't get back into the fight.
Typical scenario: Someone dies on said fight, and can't be revived but the group can live for a while, even though it's likely futile. The player that dies simply runs back into the instance. Oh noes, what's this? A wall! Now said player can't do anything but stand there.
So a way to get around this is to cast a party wide buff such as Arcane Brilliance. You will quickly notice that you are now in combat. Now this can either be used one of two ways.
Example 1: The player who died and ran back and regained combat with the boss can use any survival tactics such as ice block, or evasion, or spam heals, or bubble and can wait for the tank to get back, if of course he can make it back quickly, and hopefully you can down the boss.
Example 2: You foolishly released and forgot that you can now not receive loot, well all you have to do is get in combat with the boss and you will now be able to receive loot.
So generally you CAN'T do this on fights like this, the big gate won't allow you to get into combat, can't be blinked through or anything else. If Blizzard wanted you to get back into combat with the boss, there wouldn't be a door.
I hope this can help some people, I know it's not an epic exploit or anything but it should be able to save people a few wipes / loot lossage. Any questions or feedback is appreciated!