There's no practical way to do this unfortunately. Setting items to plot would create other issues.
How does storage work? Does it change the name, weight and set the value of the item to zero? Or replace it with an entirely different item?
If it does alter the properties of an item, rather than replace it, could part of that script not be used to force a value to zero? Even if it has to be done through an npc? E.g an engraver or something, to mark it as "your own".