@CrustyBaker, you are doing something custom so a bit of customization is needed to achieve this. The plugin can be modified so this registration number (28770) can be added to the custom field of PayPal. This will allow the plugin to add this number to the email that is sent to the customer after purchase.
Before I can give you a solution I need to know how this registration number (28770) is generated. Can you please explain that?