Many years late to this thread. I have the same issue, and thought I should pass along my solution. My production order has 3 operations. The external operation is the middle operation, and I issue my components to the first operation. So, the second operation has no material components to send. I solved this in the BOM by adding a text item as a proxy for my physical WIP item which gets processed externally. I removed the normal "Fixed quantity" flag for text items and gave the text "un-treated sub-assembly".
I then allocated this text item to my external processing operation. When I process my purchase reqs into the PO, it now has a component and no more error. Also, the text item states exactly what I am sending to the subcontractor and even has the correct quantity, since I removed the "Fixed qty" flag on the BOM item.
This does not help me with shipping paperwork, but the printed purchase order and a manual BOL are sufficient for my needs. The subcontractor is just a few miles away.
I hope this helps someone.