Can you kindly reply if your query is answered.
I think Daniele solution with solve your issue.
You have to create the following step :
1. Create a sucontracting inforecord (type 3).
2. Create a rounting with external operation ( control Key PP02) and the subcontracting data contains in the operation details.
3. When you create a production order the system create a purchase requisition subcontracting with the master data inserted before.
4. When you release the operation and create purchase order subcontracting the system required mandatory at least a component for this external operation.
In this case you can insert in the bom a text position ad attribuite this at external operation, avoiding to create a dummy material.
The text item will be inserted in the position component of the purchase order subcontracting and it won't create problem during good issue and good receipt.