Custom Event - PO Buyer as Invoice Recipient

Has anyone created a custom event that would, upon creation of an External Supplier Invoice, do the following.

1. Look at PO Reference field on External Supplier Invoices screen and identify the value in BUYER field, within the header of the purchase order.
2. Update the INVOICE RECIPIENT field of the External Supplier Invoice with this value.

Appreciate an invoice can contain multiple purchase orders. If the event ignored these  or obtained the buyer from the first purchase order on the invoice that would suffice.

Any thoughts or guidance appreciated.