It could be a nice twist, but my gut tells me that they are on the Lightside. After all, they help Rand & Co. very much through the series (and another Verin-like reveal would seem a bit boring). I think there is also nothing really to suggest that one of them is a DF, especially can the Elza and Domination Band-thing be explained in other, more convincing ways, as pointed out by the others.