So after doing a custom backpack, I gave it the option to attach the "Headgear" and "Mask" inventory slot, this works but only if you dont have a mask/helmet attached to the survivor them selves.

Also if you have a helmet/mask on the character, and you have your backpack on the floor with a mask/helmet attached too it, you cannot wear the backpack due to the limitations (this use to work prior to the dayz updates)