Naked warriors aren't exactly commonplace, the location of training should be irrelevant. By that logic, Celts and Germanics should be completely immune to their effects, which would put into question why such a unit even exists.

I think the current system of morale works as well as they can get it.. Gaesatae can route a group of levy spearmen after a short bit of combat. Against career soldiers, they have a tougher time, but can still make it through, while they are only effective against elites because of their two hitpoints.