Why do you (teh plural form^^(its really weird thing in english you sometimes cant know)) need to need a custom name for a sack while RPing? Can't you just pretend it is this? Simply because you need a proof, while showing it to others.
But there is a problem. Any person may call an item exactly the same way as your given item. So there is no point in custom name for an item, because such an item has exactly the same meaning as item without custom name. You have no proof eighter way. So things may be really the way they are now. Unless you have poor memory and you need different names to remember what this thing is, only i can't see why you need to rename instead of add player made description which should be reed instantly, by any RPer finding such a item. Only don't expect you can make claymore from a dagger. All descriptions, which don't change the provided by game mehanics item, but extend it in valid way is ok. Other variations aren't ok at all. If you don't agree, you don't need 3D enviroment or game mehanics at all, but a GM.