Reviving this thread so that it can be answered more thoroughly.

An example of a mask such as the one on the prodromos can best be seen on a relief from Pergamon, which shows a pile of Seleukid military equipment. I'll be damned if I can find it easily online, but you can best see it in a few Osprey publications. For example, the fellow on the far right is wearing not only the helmet and mask, but also the arm protectors, sword, and linothorax from the relief. More importantly, a prodomos could afford a mask with his helmet, and, considering his role to chase down skirmishers and skirmishing cavalry (units which throw things), a mask to protect the face from a thrown projectile would probably be desirable. Plus, it could come handy in a cavalry melee.

So yeah, just didn't want this to go unanswered since I recently came upon the specifics.