I have not heard of any plans to add cost to an order's line item.
However, you could definitely perform this mapping as an app by verifying costs in response to an order webhook.
Additionally, for orders that existed before your app was installed, you could compare the updated_at fields on both the order and the inventory_item to determine if the cost has potentially changed since the order was placed.
Thanks Alex, I thought about that but it's really not ideal.
What we want is line_item.cost, just like line_item.price
My understanding is that the Shopify margin reports are using the historical cost of the line item (at the time of the sale), so I would assume that it is already captured somewhere in the order data.
Consider this an official feature request :)