As we all know, when you defeat Diablo it takes a while for him to actually die. So, once he is defeated, have your friend(s) join your game and spam click your teleport banner.
It will give them the old "You cannot join during a Boss Encounter, blah blah blah," but if done correctly, it will teleport them to you just as he officially dies, allowing everyone access to the loot.
This is working as of 6/28/2012 at 1400 CST, as long as it's done properly. The trouble is finding enough elites in Act 4 to gain 5 Nephalem Valor stacks, though changing to MF gear prior to the defeat isn't too hard.