Individual Draft and AR invoices based on customer PO

We are on R12 using Project Costing and Billing.  We are doing a lot of billing by manual event and also need to invoice our customer by individual PO.

Creating separate Agreements will not work for us.

We would like to enter the customer PO in a DFF on the Event form.  Based on the unique PO in the DFF we want to create separate PA Draft Invoices and AR Invoices.

Can a Billing Extension accomplish this or do we need to modify the seeded PA Draft Invoice process?

Any help is appreciated.