I imagine that one comment you'll get- so I'll jump in first and get it out of the way!- is that it is impossible (from a rules standpoint) for an umpire to get hit by a "foul tip"!
Beyond that, I don't really do much slow pitch, probably only about three dozen games in my entire career, and am curious to see what others have to say.
Besides taking one off the nose, wouldn't you have the same concern with a foul ball (not a foul tip!) catching you in the collar bone or sternum? I cannot recall ever seeing a slow pitch umpire wearing a mask, but could understand the urge to use one. Even a lightweight chest protector- something like the Honigs softball model- would offer some protection. And what about a cup!
Or is there some unwritten slow pitch rule that would put your manhood into question for wearing these safety devices?