So lets say you have 400 people who want to do Sha of Anger for an example. It is possible to have everyone have a "tag" on the world bosses and qualify for loot. Scenario laid out below..
a) RAID A has 40 people and engage boss.
b) 9 people from raid A leave and form their own group
c) RAIDS B-J form and each fill with 40 people and hit the boss.
Congratulations, 400 people got loot from one spawn. Working as intended?
If not, easy fix - only allow the initial group of people who tag any world boss quality for loot (snapshot taken on entering combat).
But if its working as intended, gratz, everyone enjoy your loots!