It would be so easy to implement and would be something that could be a challenge depending on what the modifiers are. Lets be honest, most of the 315-320 players can do the raid in their sleep with a competent fireteam. Adding this might spice things up a bit and add another way people could get that last item they need to reach 320.
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Does nightfall completely block out people without the required light level ? If not then then you know people will abuse this Get 5 guild friends to carry their lowbie character through for that guaranteed epic drop. Rinse and repeat each weak and you turn a fresh level 40 to max light in a month of little work