Yeah 16 people with the mask equipped already tried - zilch, nadda, zip.
Now about that amulet I heard of - has anyone tried this with the Amulet?
The only theory that I know of is that In 16 man Nightmare there is a bonus boss in the cave which requires all raid members to equip their Dread Guard Helm (the one that drops from Dreadtooh), as it protects them from "Dread Touch." The mask should work just like the Gormak Crystals work for Nightmare Pilgrim protecting you against Mental Assault. The cave is not currently large enough to fit any kind of boss, which only leaves room for speculation that it is a Nightmare only mechanic and therefore not currently implemented.