Someone revealed it.
Info here:
http://oldschoolrunescape.wikia.com/wiki/Helm_of_raedwald
Tl;dr In order to obtain the helmet, the player must dig between the four iron rocks by the cave entrance east of the Clock Tower while having a spade, a nature rune, a superantipoison with one dose remaining, and a pair of leather boots.
How it was solved: