Where does this custom inventory data "live" at runtime?

[anonymous_user_741503d3]

But a designer can’t, to my knowledge, make a blueprint class derived from a struct. A struct isn’t an entity on its own, so how would a designer create and define the items from that base struct?

He can’t indeed. In my case you store this struct on a pickup. So designer creates an object ONE time, both for pickup and for item but to him it looks like a pickup. His inventory is a TArray<FInventoryItem> and when he picks up an item you just generate it using the data you have. In my case it’s just a Generate() method on a pickup which creates an item from itself which I then can add to player’s inventory.