It would have been a Great Hau of some sort, but I really can’t decide on a shape.
From a real world parts production perspective, if Lego chose to make a Toa Jaller wearing a Hau in 2006 or 2007 they probably would have gone with the Lhikan Hau, since we know it was still available in 2006 when it was used in the Duracell Gold Good Guy (Turaga Lhikan) set. Its possible the Gold Good Guy though used excess stock of the mask since it used the old gold colors and not the pearl gold that late production 2005 and all the 2006 sets used, but it still has a better chance the mold existed at that point. Also the axle on the Lhikan Hau is a more snug fit on a Metru Head. Unfortunately by 2006, I have doubts that the original Hau mold from 2001 was still available since it had been last used in 2003 and had been out of use for the three years after that (and when the original Hau eventually did return it was with a new mold and design style in 2010)