I believe that the introduction of inventories happened somewhere in the last 10 months. Hearthlings can now carry multiple items at once, like the trapper could since the beginning.
Therefore, the “need” for something like this has somewhat vanished, assuming the AI is capable of doing some sort of clever salesman (which I’m not sure if it’s currently implemented).
The problem with portable storage is, and always will be, that it’s wacky. You would need to explicitly say that other storage cannot fill the cart (otherwise they’d empty the stockpiles into the cart) for starters. I don’t know if it changed - I’m really not uptodate - but the main issue could be that inventory effectively worked reversely from what you thought it does. It’s not the hearthling or even the item searching an inventory to be placed in; it’s the inventory “calling out” to the item.