I have a reference here for a Hamo de Mascy who is alive in 1356, going to war:
from an article about the Chesire Archers, which conflicts with the information about thie death of this Hamo de Mascy:
http://www.createwrt.net/Irene_Prestons_Historical_Writing/CHESHIRE...
There seem to be multiple Hamo de Mascy's at this time, I think some of the related families named their sons after the Barons of Dunham Mascy. One cited is the son of Robert of Roshtherne. The family seems to be very involved with the Black Prince and his Wars, which makes sense as they and their kinsmen were supplying most of the Chesire Archers for the Prince's battles.
Another reference:
19H(M). HAMON MASCY, de PONINGTON, lived in Ponington, England. Was son of Thomas Mascy and Agnes de Leigh. Knighted by 1366. Hamon fought for the Black Prince at the Battle of Poitiers in France in 1356, as reward for which he received pardon for having slain some men in Cheshire, except for his responsibilty for the death of the king's minister, Bertram de Norden, and that of Richard de Bechyngton. One is left to wonder concerning the punishment, if any, for the slaying of the two as to which his pardon did not extend. Died c1380.
There is a Hamon Mascy who was born in 1284 in Tatton, Cheshire, England. It is possible this is the same Hamon Mascy. His father was listed as Robert Massy
Source:
http://www.kenworthypast.com/#/normanmassey-lineage/4535196050