At version 8, reports can be customized within the User Interface just like any other view.

Create a new view that inherits the report_picking report view and adds the information you want:

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(Here is the raw text so you can copy and paste:)

<?xml version="1.0"?>

<!-- specify the anchor that the change will appear after --> 
<span t-field="o.name" position="after"> 

  <!-- make sure client_order_ref has a value, or do nothing --> 
  <div t-if="o.sale_id.client_order_ref"> 
     Your PO#: <span t-field="o.sale_id.client_order_ref"/> 
  </div>

</span>