You optimal solution would be as BigLouie suggests - If you have work order (or as it is known now Production Management) create a outside process step, issue a purchase order (assign the line to the WO and step number) for the outside process.  When the item is returned, process a receipt of goods/invoice.  Cost will be assigned to the WO.  If you don't have WO/PM, then create an outside process inventory item (inventory valuation of lot or serial number).  Issue the PO.  Receive it back in assigning the the lot/serial number.  When you process the BOM, include that process item code and use the lot/serial item.

You optimal solution would be as BigLouie suggests - If you have work order (or as it is known now Production Management) create a outside process step, issue a purchase order (assign the line to the WO and step number) for the outside process.  When the item is returned, process a receipt of goods/invoice.  Cost will be assigned to the WO.  If you don't have WO/PM, then create an outside process inventory item (inventory valuation of lot or serial number).  Issue the PO.  Receive it back in assigning the the lot/serial number.  When you process the BOM, include that process item code and use the lot/serial item.

Another idea is to transfer the items being worked on to a separate warehouse, and do a BoM Production entry when you receive them back to add in the extra costs (GL clearing) and move them back to the normal warehouse code.  AP invoice against the clearing account and you're done.

Edit:

You probably won't want to use this strategy if you have high volumes with multiple service providers because reconciling the clearing account would get messy.