Hi Philip,
I have an idea to do it.
Create an extension field( Customer Part No) in product master level and fill this field with the customer part no apart from the original material ID. Extend the same to the screens you may work and also to output forms as well.
Extend this field to all the screens and output forms, so that this will reflect in the output forms. Here you can see 2 fields one is of original material ID and another one is of customer material ID.
Hence the person who is going to pick the material will have 2 ids in hand and the inventory will be maintained in the Original Material ID(ABC) itself.
Just check this work around, it may help you.
If you dont need the Original Material ID(ABC) to reflect in the output forms, then create 2 forms one is with the Original Material ID(ABC) and another one is without Original Material ID(ABC). So that before releasing any transactions we can choose anyone of the document be clicking the Edit Output Settings.
Regards,
Samraj