I don't have an answer, but a couple very basic questions to jump start the discussion.
Do you mean orders with no value in VBAP-LGORT?
How does this field normally get populated in your environment?
Is this field part of the incompletion procedure?
Are you doing availability checking?
If it's a small number of customers, can you use the Line Item view of VA05 (or VA35) and add the Storage Location and the Order Quantity and Net Price to start to get a handle on the value? Or something similar, like a QRY