Hey farski,
I've been able to tackle this issue through API automation. To extract the Inventory ID for the specific products ordered, you need to pull the Order from the Order API and parse the result, specifically (per order row), look at '{lineItems}{<index>}{id}'. I use Python to loop through the products and flatten the structure before import into our backend system, so i do not use the <index> field, I just throw it into a loop that hands me the list of items and I parse the fields I want.
Once you have the lineItem ID, you can send that through the Inventory API,