customize soldier init with any weapon and attachments (that are not listed as options in the cfgweapons combinations (i.e. not srifle_lrr_sos_f but instead srifle_lrr_f w/optic_dms attached))
create trigger with delay of 1 second or more with on action:
soldierOne action ["DropWeapon", unitBackpack soldierOne, currentWeapon soldierOne];

as another way of looking at it, if one wanted to customize a weapon's attachments within a container, he or she would have to addprimaryweaponattachment while on a person and then transfer the weapon to the container. why not have a command instead allow attachment to a weapon within a container like a backpack, groundweapon holder, or ammo box.