From what I've heard, it isn't the ears that are the problem so much as the hair that Viera (and I'm getting Hrothgar) have. We've seen a few hats and helmets work for Viera, but in all of those cases, either the hat is really small and not obscuring anything, or it's a fully enclosed helmet (usually a newer one), that completely sidesteps the problem.
If I had to guess, it's because a lot of helmets despawn hair to some degree to avoid clipping, and that gets rid of the Viera ears as well, but unlike say...miqo'te, none of the old legacy gear has instructions to make little bunny ear warmers.
