I predicted the NFL would have U in a hockey helmet in the not so distant future.
As an umpire, I think that's unlikely. It can be a dangerous position, but the best thing you have going for you to avoid injury is your awareness of what's around you. A helmet, especially a hockey helmet is going to adversely affect that awareness and could lead to more serious collisions.
I'm hit weekly in the middle, but I've only been rolled up twice. And there are U's with a much greater feel for the position than I. My worst collision as an official came as a H. I retreated on a 1st down play to my side and the stick man raised the chain under my foot. When the dust cleared, I was beneath a 4-player pile on the sideline. I'll take the middle of field amongst fairly predictable player movements over some yahoo on chain crew all day long.
BTW, the only person that received medical treatment after the collision above was the stick man. It seems he received a scratch on his face when some fat official planted the stick on his head while being run over. Pure justice.