I think that the fine drawing by Fattori of a drummer that Ralph has posted below does indeed depict a Frenchman but not, I suggest, at Mentana. This looks to me like a preliminary sketch for Fattori’s famous painting The Italian camp after the battle of Magenta, now in the Gallery of Modern Art in Florence. Look at the group of figures in the bottom right hand corner of the picture.
French line infantry drummers at Mentana probably wore the chasseur style uniform, shown in this watercolour by Lalaisse.
This uniform, introduced after the 1859 war, was abolished in December 1867, the month after Mentana.