I have just spent an hour fruitlessly scouring Poser manual, Poser room help and Poser menus for this fabled 'metallicity'. Upside, by serendipitously right-clicking in 'wrong' place, a context-sensitive menu appeared and, following your bread-crumb trail, I found it.

Chapter 15, Pg.397 'Metallic' not 'Metallicity', as in

"You dont need a reflect node with Superfly, that's Firefly. Use a physical surface root and set metallic value on mirror material to 1."