Hi Daniel,
If your sales order created/modified by an incoming IDocs, then you can update the Customer PO field. Provided that is coming in incoming IDoc.
Else, if your sales orders are created manually. Then ideally, it doesn't make sense to have this field auto-populated. As your client's customers is suppose to provide their PO number. In turn, this will help your client and their customer to track the origin of incoming orders.
Now, if customer your customer want this field to be auto-populated. Then you have to get this enhanced.
Thanks, JP