A pretty good idea.But the only problem with this is still the % Chance that someone can stroll in and hit an NM and still have a chance at the item. Sure it may only be ~5% chance (if they came really late), but that % Chance will eventually roll in favor of that person / LS.
It's just frustrating if you're on the "main group" that, say, did ~80% - 90% of the damage on an NM, and someone else strolls in, adds in some damage late and gets the low percentage chance roll. Sure they lucked out, but I can see this becoming an exploit of "NM Hunters" that stroll in late, and fight a few minutes, hoping to get a lucky roll, and repeat. Just feels like they didn't really earn the right to get a rare drop that way.