In the Lone Wolf and Cub series there is a clan (Kuroda, IIRC) that wears a mask with a moustache on it to intimidate their enemies. I can imagine it would have been costly to craft masks like these for the common soldier, so who exactly was allowed to wear them? The clan leader and his commanding officers?
The menpō or menbō is a pretty common part of the tōsei gusoku. Which means it would not have been rare to find mid and upper samurai wearing them in the late Sengoku.
Of course most of an army was probably ashigaru, who wouldn't be wearing one. But commander or daimyō or not, if it was a gathering of samurai in armor it was probably not rare to find people with them.