Yeah we've had a couple. Generally people try to spawn as many guards as they can. We had one group hold the Shrine of the Seven Stars bank for around 20 minutes before someone figured out that guards don't just spawn if the attacker is next to an NPC but if any friendly player is in combat near an NPC.. they ran through the shrine and spawned about twenty guards.
It's not impossible with enough healers, of course.
Also as someone mentioned earlier, the battle fatigue debuff combined with the 50% mortal strike from the guards really hurts tanks. I initially tried this on a blood DK but between the fact that the guards appear immune to dodge and parry, and with all the snap kicks and debuffs it just doesn't work.
I'd like to see what a group of stealthies could do.