From the looks of things, this appears to be designed to only show information from the order_items table which can only hold references to products which are either serialized or tagged thus items.

I am taking a look at this further but suspect this would be a change request.  I understand your need is you want to force the technician to insert the object of service item information (serial number) by only giving them the product without a serial number to ensure the work order references the correct item as opposed to pre-populating the serial number when sending the work order to the field engineer.  This pre-populated serial number may not be correct and field engineers may not always update once they see something is already populated is the rationale.